Light scanning apparatus and image forming apparatus

ABSTRACT

A light scanning apparatus that scans a scanned face with a light beam includes an adjusting unit that adjusts the position of a light spot of the light beam formed on the scanned face, and a compensating unit that compensates the light intensity of the light beam at the scanned face due to change caused by the adjustment of the position of the light spot. Accordingly, the light scanning apparatus can reduce or eliminate the deviation in exposure between scan lines of the multi-beam scan method and the deviation in exposure between photosensitive bodies of the tandem type image forming apparatus.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a light scanningapparatus, a light scanning method, and an image forming apparatus.

[0003] The present invention relates more particularly to a lightscanning apparatus (multi-beam scanning apparatus) that simultaneouslyscans a face (scanned face) with a plurality of light beams emitted by alight source(s), and to an image forming apparatus such as a laserprinter, digital copier, a laser facsimile, and a laser plotter usingthe light scanning apparatus to write images.

[0004] 2. Description of the Related Art

[0005] Light scanning apparatuses are used for laser printers, digitalcopiers, facsimile machines, and laser plotters, for example. The lightscanning apparatus conventionally uses one light beam (single beamscanning method), but the light scanning apparatus that uses a pluralityof light beams to simultaneously scan a single scanned face (multi beamscanning method) is being studied intensively.

[0006] Tandem type image forming apparatuses with a plurality ofphotosensitive media that can form color images are also being studiedas well as image forming apparatuses with a single photosensitivemedium. The tandem type image forming apparatus is provided with aplurality of drum-shaped or belt-shaped photoconductive photosensitivebodies along the path of a medium to which toner images are transferred.The toner images formed on the plurality of photosensitive bodies aretransferred to the common medium (recording sheet) and synthesized intocolor images. The tandem type image forming apparatuses of themulti-beam scanning method are being studied too.

[0007] A scan line is the trace of the light spot scanning the scannedface. The ideal scan line is a line straight in the main scandirections, but the actual scan line is curved and/or tilts even if thelight scanning apparatus is precisely assembled. Since the tilted scanline can be regarded as an aspect of the curved scan line, the curvedscan line includes the tilt scan line in the following description.

[0008] In the case that a monochrome image is formed with the singlebeam scan method, even if the scan line is slightly curved, thecurvature causes few problems since the slight curvature is not visible.

[0009] In the case that a scanned face is scanned with the multi-beamscan method, if all the scan lines curve in the same manner, thecurvature causes few problems.

[0010] However, if the distances between a plurality of scan lines (scanline pitch) differ, the curvature does matter. If the scan line pitchwithin an image varies in the sub scan directions, the image isdistorted or the density of the image is not uniform. The distortion ofthe image and the non-uniformity of the image density degrade thequality of the image substantially.

[0011] If the curvature of the scan lines is not uniform in a colorimage formed by a tandem type image forming apparatus, the curvatureresults in an unevenness in color, density, and hue.

[0012] In the case of the multi-beam scanning method, the scan pitchchanges over time however precisely the light scanning apparatus isinitially adjusted. In the case of the tandem type image formingapparatus, the change over time in the curvature of the scan lines isinevitable even if the curvature is equalized at the initial stage.

[0013] Especially, if a resin lens is provided in the light path fromthe light source to the scanned face, the resin lens may deform due tochanges in temperature and/or humidity. Accordingly, the scan line pitchin a photosensitive body may change, and the curvature of the scan linesof different photosensitive bodies may differ.

[0014] The scan line pitch and the scan line curvature can becompensated by adjusting the light spot position on the scanned face inthe sub-scan directions. There are various methods to adjust the lightspot position.

[0015] Japanese Patent Laid-open Application No. 9-189873 discloses aninvention in which the scan line pitch of the multi-beam scanning methodis adjusted at high precision by reflecting light beams emitted by lightsources with a galvanic mirror on the light path to a light deflectingunit and adjusting the angle of the galvanic mirror to move the lightspot position on the scanned face in the sub-scan directions.

[0016] The above invention also proposes to compensate the scan linecurvature and the scan line pitch by adjusting the light spot positionin the sub-scan directions on the scanned face with a liquid crystalelement.

[0017] The above methods effectively adjust the scan line pitch andcompensate the scan line curvature, but inherit a side effect in thatthe adjustment of the light spot position also changes the lightintensity of the light spot.

[0018] In the case of the invention described in Japanese PatentLaid-open Application No. 9-189873, if an aperture for shaping the lightbeam is provided between the galvanic mirror and the scanned face, thelight beam adjusted in the sub-scan directions by the galvanic mirrormay be partially blocked by the aperture, and consequently, the lightintensity of the light spot may be reduced.

[0019] In the case that the liquid crystal element is used, a change indeflecting angle causes a change in the transmissivity of the liquidcrystal element, and results in a change in the light intensity of thelight spot.

[0020] In the case of the multi-beam scanning method, for example, ifthe light intensity of the light spots varies between the scan lines,especially the quality of half-tone images often degrades substantially.In the case of forming color images of the tandem type, if the exposureof the photosensitive bodies differs from one body to another due to theinequality of the light intensity of the light spots, the hue of thecolor images changes, and accordingly, the color reproducibility isdegraded.

[0021] One of the methods to increase the writing speed at which thelight scanning apparatus forms images is to increase the rotative speedof a polygon mirror that is a deflecting unit. This method has a limitin applicability due to the durability, noise, and vibration of a motorthat rotates the polygon mirror and the limit in the modulation speed ofthe laser. To solve the above problems, a proposal is made for a lightscanning apparatus that writes a plurality of lines simultaneously usinga plurality of light beams (see Japanese Patent Laid-open ApplicationsNo. 2000-227563, 10-215351, and 9-189873, for example).

[0022] A multi-beam semiconductor laser is, for example, a semiconductorlaser array in a package that emits a plurality of light beams. Such amulti-beam semiconductor laser can be used as a light source unit of thelight scanning apparatus (multi-beam scanning apparatus). In the case ofthe semiconductor laser array, however, it is difficult to increase thenumber of channels due to the limitations of fabrication processes, toremove thermal and electrical crosstalk, and to shorten the wavelengthof the light beam. The cost of the semiconductor laser array is stillhigh.

[0023] On the other hand, single beam semiconductor lasers are used forgeneral purposes in various industrial fields since short wavelengthtypes are readily available and production cost thereof is low. Manyproposals have been made for multi-beam scanning apparatuses that usethe single beam semiconductor lasers or the above multi-beamsemiconductor lasers as the light source and generate a plurality oflight beams using a beam generating unit.

[0024] In the case that a plurality of light beams are generated usingthe beam generating unit, compared with the case in which thesemiconductor laser array is used as the light source unit, the beamspot arrangement (scan line pitch) on the scanned face often changes dueto environmental and timewise (elapsed time) change.

[0025] Accordingly, a method of compensating the beam spot arrangementon the scanned face by providing an electrically driven liquid crystalelement in a light source unit or just after the light source unit, anddeflecting the light beam in response to the electrical signal by amicro angle (from several minutes to several tens of minutes) isproposed (see Japanese Patent Laid-open Applications No. 2000-3110 and2000-47214).

[0026] A description is made of the liquid crystal below.

[0027] The liquid crystal element used as the light path deflectingelement includes a nematic liquid crystal layer of homogenous moleculararrangement sandwiched by two glass substrates. Inside of the glasssubstrates are formed transparent electrodes made of metal oxide.Generally, a uniform electrode is formed over the entire surface of aglass substrate (bottom face, for example) to form an electrical groundplane, and a patterned electrode that provides electric fielddistribution to the liquid crystal layer is formed on the other glasssubstrate (top face, for example). When an alternating voltage(rectangular pulses of several kilo Hertz, for example) is applied tothe liquid crystal layer, nematic liquid crystal molecules havingbirefringence (a difference in refractive index along the long axis andthe short axis of the molecules) are tilted along the electric field.The liquid crystal layer is equivalent to a medium having a locallydifferent refractive index distribution in response to the electricfield distribution for a single color light having a linear polarizationparallel to the liquid crystal molecule (the direction of the longaxis). Accordingly, the light transmitting threough the liquid crystallayer is spatially modulated in its wave surface or phase depending onthe in-plane distribution of the applied voltage.

[0028] The electro-optic property of the liquid crystal element dependson the elasticity and the dielectric anisotropy of the liquid crystalused and the initial orientation angle of liquid crystal moleculeswithout the application of voltage. The electro-optic property of aliquid crystal element with a small initial orientation angle (5 degreesor less) exhibits a steep rise (threshold) in a low voltage region. Asthe voltage increases, the electro-optic property becomes linear, thenconverges into a constant. The electro-optic property of a liquidcrystal element with a large initial orientation angle has no threshold.The curve in the low voltage region can be approximated with a secondorder polynomial.

[0029] A proposal is made on the pattern of electrodes in which manylong and narrow stripe-shaped electrodes are provided and apredetermined voltage is applied to each electrode. This structure ischaracterized in realizing a high speed response, a high spatialresolution, and degrees of freedom in the wave surface modulation (anycomplex wave surface modulation as well as the functions of beamdeflection and lenses).

[0030] In the case that the light path deflecting element is constructedby the liquid crystal element, the linear region of the electro-opticalproperty of liquid crystal and ladder shaped electrodes are used. Thestripe-shaped long and narrow transparent electrodes of the currentexposure technique having width and pitch depending on the resolution(about 1 μm) are formed in the beam exposing region. Both ends of thestripe-shaped electrodes are connected to gradient potential electrodesexpanding horizontally on the outside. The electrodes are structured insuch a manner that a plurality of ladder-type electrodes are arranged.The number (width) of bound long and narrow electrodes is determined bythe maximum beam deflecting angle required in the region.

[0031] In the case that two different voltages selected from the linearregion of the electro-optical property are applied to respective ends ofthe gradient potential electrode spreading in the crosswise directions,a blaze type phase profile is obtained and becomes equivalent to a microprism array. The deflecting of the light beam perpendicularly incidenton the liquid crystal layer is possible by controlling the appliedvoltage, and consequently the blaze angle.

[0032] In the case that the liquid crystal element is used as a lightpath deflecting element, as described above, it is necessary to providean electric field distribution to the liquid crystal layer and to form alinear refractive index distribution (refractive index gradient). On theother hand, in the case that a large beam deflecting angle (maximumdeflecting angle) is desired, the thickness of the liquid crystal layerneeds to be increased. There is no spherical spacer material to bedistributed in the liquid crystal layer for general use. The thicknessof the liquid crystal layer for liquid crystal monitors is usually 5 μm.In the case that a liquid crystal layer with thickness of more than 10μm, for example, needs to be secured, one needs to use low-grade spacermaterial with high diameter deviation. As a result, it is difficult tokeep uniform thickness of the liquid crystal layer, and to sustain thelinearity of the refractive index distribution of the liquid crystallayer, for example.

[0033] A tandem type full-color image forming apparatus is provided withfour photosensitive body drums corresponding to cyan (C), magenta (M),yellow (Y), and black (K) disposed along the transportation surface of aintermediate transfer belt. A light scanning apparatus correspondinglyprovided for each photosensitive body drum scans the photosensitive bodydrum. An electrostatic latent image is formed on the surface of eachphotosensitive body drum, and is made visible with a toner ofcorresponding color. The toner images are sequentially transferred to asheet of paper carried by the intermediate transferring belt, and amulti-color image is formed.

[0034] The scanning unit of the above light scanning apparatus isusually a polygon mirror rotated by a motor at a predetermined rotativespeed. The light scanning apparatus includes a line cycle signalgenerating unit. The line cycle signal generating unit detects the laserbeam from the scanning unit at a predetermined position, and generates aline sync signal. The laser beam is modulated by the image signal insynchronization with this line sync signal, and the image is writtenline by line. An intermediate transfer reference signal generating unitdetects a mark on the intermediate transfer body at a predeterminedposition, and generates an intermediate transfer reference signal. Animage forming operation of each color to form a toner image of the coloron the photosensitive body drum is executed in synchronization with theintermediate transfer reference signal.

[0035] In such a color image forming apparatus, since the intermediatetransfer reference signal and the line sync signal are not insynchronization, the phases of the intermediate transfer referencesignal and the line sync signal greatly deviate as the number of thelaser beams increases. Since the starting positions at which the imagesare written in the sub-scan directions deviate, the position of thetoner image of each color deviates from the others, which results in thedegrading of the multi-color images.

[0036] To solve this problem, a proposal is made on a color imageforming apparatus characterized by a compensating unit that adjusts thestarting position at which the image of each color is formed in thesub-scan directions and compensates for the color image deviation byswitching the laser beams that first write the images on thephotosensitive body drum depending on the phase relationship of theintermediate transfer reference signal and the line sync signal (seeJapanese Patent Laid-open Application No. 10-239939).

[0037] However, the order in which the laser beams start writing changesrandomly depending on the phase difference between the mark signal andthe sync detection signal attached to the intermediate transfer belt.Accordingly, even in the case that the difference between the power ofthe plurality of laser beams is very small, the light energy exposingthe photosensitive body drum of each color varies, even though the imageof the color is the same. Accordingly, the color of the image becomesunbalanced (see Japanese Patent Laid-open Applications No. 2002-72606and No. 2002-72607). According to an experiment, the color variationcaused by a power deviation of 2% is already visible. The colorvariation is especially apparent in the case of reproducing grey color.

[0038] On the other hand, the image forming speed of such a color imageforming apparatus needs to be improved. It is necessary to increase therotative speed of the polygon mirror that is a scanning unit of thelight scanning apparatus and/or to increase the frequency of the imagesignal in order to satisfy the need. However, if the rotative speed ofthe scanning unit and the frequency of the image signal are increased,the durability, noise, and vibration of the motor driving the polygonmirror and the modulation speed of the semiconductor laser causeproblems. Accordingly, a multi-beam scanning apparatus is proposed thatsimultaneously scans a plurality of light beams and writes a pluralityof lines.

[0039] The multi-beam semiconductor laser such as the semiconductorlaser array that has a plurality of radiation points (radiationchannels) in a package can be used for the multi-beam light sourceapparatus that emits a plurality of laser beams. However, it isdifficult to increase the number of channels due to the restrictions offabrication processes, to remove thermal and electrical crosstalkbetween channels, and to fabricate the semiconductor laser array thatemits light beams of short wave length. Multi-beam semiconductor lasershaving such desired features are expensive.

[0040] Light source apparatuses and multi-beam scanning apparatuses thatare provided with single-beam semiconductor lasers as the light sourcesand generate a plurality of laser beams using a beam generating unit arealso usable. The plurality of laser beams generated by the beamgenerating unit are often affected by environmental and changes overtime. The arrangement of beam spots on the scanned face (beam pitch)consequently changes. To solve this problem, a method is proposed inwhich an electrically driven liquid crystal element is provided tocompensate the beam pitch.

[0041] However, the liquid crystal element to adjust the light beampositions of the plurality of light beams on the scanned face causesdeviation in intensity of the plurality of light beams. This deviationmay result in the degrading in quality of images formed by the colorimage forming apparatus.

SUMMARY OF THE INVENTION

[0042] Accordingly, it is a general object of the present invention toprovide a novel and useful light scanning apparatus and image formingapparatus with which at least one of the above problems is solved.

[0043] More particularly, it is an object of the present invention toeffectively compensate the light intensity deviation of the light spotaccompanying the compensation of the scan line pitch and the scan linecurvature including tilt.

[0044] Another object of the present invention is to provide amulti-beam scanning apparatus that can adjust the light beam position onthe scanned face without affecting various optical properties thereofand an image forming apparatus using the same.

[0045] Yet another object of the present invention is to provide a lightscanning apparatus that can reduce the deviation in intensity of theplurality of light beams on the scanned face, and an image formingapparatus using the same.

[0046] To solve at least one of the above problems, a light scanningapparatus that scans a scanned face with a light beam, according toclaim 1, includes: an adjusting unit that adjusts a position of a lightspot of said light beam formed on a scanned face; and a compensatingunit that compensates a light intensity of said light beam at saidscanned face caused by the adjustment of said position of said lightspot.

[0047] The adjusting unit adjusts the position of the light spot on thescanned face, and the compensating unit compensates the change inintensity of the light spot caused by the adjustment of the light spotposition by the adjusting unit.

[0048] The light scanning apparatus according to claim 2, may be furthercharacterized in that said light scanning apparatus scans said scannedface with a plurality of (N) light beams emitted by “N” light sources;said adjusting unit further comprises at least “N−1” deflecting unitslocated between said light source and a scanning unit, each of thedeflecting units deflects corresponding one of the plurality of lightbeams in sub-scan directions and adjusts scan line pitch.

[0049] In this case, assuming that each light source is a semiconductorlaser emitting a light beam, the scanned face is simultaneously scannedwith N light spots and N scan lines are formed. There are “N−1”distances between scan lines to be adjusted. All distances between scanlines can be adjusted by deflecting “N−1” light beams in the sub-scandirections using a light beam as a reference.

[0050] Each of the above “N” light sources is not necessarily a lightsource that radiates a single light beam, and may be a semiconductorlaser array with arrayed “n” semiconductor laser radiation sources. Inthis case, the scanned face is scanned with “N*n” scan linessimultaneously. Regarding “n” scan lines formed by the “n” light beamsradiated by each semiconductor laser array as a group, the scan linepitches of the “N” groups may need to be adjusted. In this case, the “n”light beams are deflected by the group by “N−1” deflecting units.

[0051] Of course, if “N”, instead of “N−1”, deflecting units are used,all “N” scan lines can be displaced in the sub-scan directions. In thecase that each photosensitive body is scanned with multi-beam method ina tandem type image forming apparatus, the positions of the toner imagestransferred from each photosensitive bodies can be adjusted moreprecisely.

[0052] The deflecting units may be liquid crystal deflecting elements(claim 3).

[0053] Said deflecting unit may further include a semiconductor laserand a coupling lens combined by a holder rotatable around an axisparallel to an optical axis of said coupling lens, radiation source ofsaid semiconductor laser being eccentric to said optical axis (claim 4).

[0054] Said deflecting unit may further include an aperture combined bysaid holder that shapes said light beam, said aperture being concentricto a light path of said light beam emitted by said semiconductor laserand passing through the center of said coupling lens (claim 5).

[0055] Said adjusting unit may further include an liquid crystaldeflecting element array having a plurality of liquid crystal deflectingelements arrayed in main-scan directions, each of which deflects saidlight beam in sub-scan directions, said liquid crystal deflectingelement array provided between said scanning unit and said scanned face(claim 6).

[0056] The liquid crystal deflecting element array can compensate thecurvature of the scan line (including tilt). The liquid crystaldeflecting element array functions as an adjusting unit that adjusts alight spot position, and effectively adjusts the curvature of the scanline on each photosensitive body of a tandem type image formingapparatus that scans each photosensitive body with a single beam method.

[0057] Assuming there are “m” photosensitive bodies, the number ofrequired liquid crystal deflecting element array is “m−1” or “m”. In thecase that using a scan line as a reference, the curvatures of the otherscan lines are to be adjusted to the reference, only “m−1” liquidcrystal deflecting element array suffice.

[0058] As described above, when the light spot is adjusted by theadjusting unit, the light intensity of the adjusted light spot changes,and the light intensity of the light spots loses uniformity. Thecompensating unit compensates this non-uniformity.

[0059] In general, in the case that the light spot position is adjustedby the adjusting unit, it is possible to determine how much the lightintensity of the light spot changes as the light spot position isadjusted in advance theoretically or by experiments. The compensatingunit can compensate the light intensity based on this theoretical orexperimental knowledge. However, the actual change in the lightintensity may depend on the deviation of components and assembly, anddegrading over time of the precision of assembly.

[0060] The light scanning apparatus may further include a detecting unitthat detects intensity of said light beam (claim 7), and said detectingunit may further detect synchronization of light scanning (claim 8).

[0061] Said compensating unit can control radiation intensity of saidlight source (claim 9).

[0062] The light scanning apparatus may further include an apertureprovided between said light source and said scanning unit, that shapessaid light beam; wherein said compensating unit displaces said aperture(claim 10).

[0063] The light scanning apparatus as claimed in claim 1, is furthercharacterized in that said compensating unit controls a transmissivityadjusting unit provided between said light source and said scanning unit(claim 11). The compensating unit may be used alone or combinedtogether.

[0064] The light scanning apparatus as claimed in claim 1, may furtherinclude a resin lens provided in the optical path from said light sourceto said scanned face (claim 12). Since the optical properties of theresin lens depends on temperature and humidity, the position of lightspot may change and increase the curvature of the scan line. The resinlens, however, is useful in forming a lens of complex shapeinexpensively.

[0065] As described above, the light scanning apparatus can adjust theposition of the beam spot with the adjusting unit, and the aboveproblems of resin lenses can be solved.

[0066] An image forming apparatus according to claim 13, ischaracterized of: a photosensitive medium; and a light scanningapparatus that scans said photosensitive medium with a light beam;wherein said light scanning further includes: an adjusting unit thatadjusts a position of a light spot of said light beam formed on saidphotosensitive medium; and a compensating unit that compensates thelight intensity change of said light beam at said photosensitive mediumcaused by the adjustment of said position of said light spot.

[0067] In the image forming apparatus as claimed in claim 13, saidphotosensitive medium may be a photoconductive photosensitive body; andan electrostatic latent image formed by the light scanning may be madevisible by being converted into a toner image (claim 14). In this case,said light scanning apparatus scans said photoconductive photosensitivebody with a plurality of (N) light beams emitted by “N” light sources;and said adjusting unit further comprises at least “N−1” deflectingunits located between said light source and a scanning unit, each of thedeflecting units deflects a corresponding one of the plurality of lightbeams in sub-scan directions and adjusts scan line pitch (claim 15).

[0068] Of course, the image forming apparatus according to claim 15 mayinclude a liquid crystal deflecting element array of claim 6 between thescanning unit and the scanned face as the adjusting unit or a part ofthe adjusting unit. In this case, it is desired to provide the detectingunit that detects the light intensity of the light beam.

[0069] The detecting unit can have a function to provide thesynchronization of the light scanning as well. The compensating unit hasa function to adjust the radiation intensity of the light source, afunction to displace the aperture for shaping the light beam of claim13, and a function to adjust the transmissivity being provided betweenthe light source and the scanning unit. The compensating unit may havetwo or more of the above functions simultaneously. Needless to say, aresin lens may be provided in the light path from the light source tothe scanned face.

[0070] The image forming apparatus as claimed in claim 13, may becharacterized in that said image forming apparatus is a tandem type inwhich one or more photosensitive bodies that are drum-shaped orbelt-shaped are provided along a path of a toner image medium, and atoner image formed on each photosensitive body is transferred to saidtoner image medium generating a composite color image (claim 16).

[0071] In this case, three or four photosensitive bodies may be providedcorresponding to magenta, cyan, yellow, and black (claim 20), or insteadof the above colors, red, green, and blue toners may be used.

[0072] The toner image transferring medium is an intermediatetransferring medium such as an intermediate transferring belt or arecording sheet. The recording sheet is a transferring paper or overheadprojector (OHP) sheet, for example.

[0073] As described above, the light scanning apparatus adjusts theposition of the light spot on the scanned face and compensates thechange in the light intensity of the light spot caused by the adjustmentof the light spot position. Accordingly, the light scanning apparatuscan reduce or eliminate the deviation in exposure between scan lines ofthe multi-beam scan method and the deviation in exposure betweenphotosensitive bodies of the tandem type image forming apparatus.

[0074] A light scanning apparatus that scans a scanned face with aplurality of (N) light beams, according to claim 21, is characterized bya plurality of adjusting units, each of which adjusts a position of ascan line formed by a corresponding one of the plurality of light beams;wherein at least one of the plurality of adjusting units is a liquidcrystal element driven by an electric signal.

[0075] The invention according to claim 22, is characterized, in thelight scanning apparatus as claimed in claim 21, by a memory unit thatstores said electric signal driving said liquid crystal element.

[0076] The invention according to claim 23 is characterized in that, inthe light scanning apparatus as claimed in claim 22, said liquid crystalelement initially adjusts said position of scan line in compliance withsaid electrical signal stored in said memory unit.

[0077] The invention according to claim 24 is characterized in that, inthe light scanning apparatus as claimed in claim 21, said liquid crystalelement adjusts the position of a light beam of which change is causedby an external disturbance.

[0078] The invention according to claim 25 is characterized in that, inthe light scanning apparatus as claimed in claim 21, said liquid crystalelement is able to deflect said light beam by a micro angle.

[0079] The invention according to claim 26 is characterized in that, inthe light scanning apparatus as claimed in claim 21, at least “N−1” ofthe plurality of adjusting units are liquid crystal elements.

[0080] The invention according to claim 27, is characterized in that, inthe light scanning apparatus as claimed in claim 26, a maximumdeflecting angle of each liquid crystal element is +/−4.0 (minute) orless.

[0081] The invention according to claim 28 is characterized in that, thelight scanning apparatus as claimed in claim 21, the plurality ofadjusting units are liquid crystal elements of which maximum deflectingangle is +/−2.0 (minute).

[0082] An image forming apparatus according to claim 29, ischaracterized by: a plurality of scanned faces; and a light scanningapparatus that scans the plurality of scanned face with a plurality of(N) light beams and forms electrostatic latent images on the pluralityof scanned faces; wherein said light scanning apparatus furthercomprises a plurality of adjusting units, each of which adjusts theposition of a scan line formed by a corresponding one of the pluralityof light beams; and at least one of the plurality of adjusting units isa liquid crystal element driven by an electric signal.

[0083] The invention according to claim 30 is characterized in that, inthe image forming apparatus as claimed in claim 29, said liquid crystalelement can change pixel density in sub-scan directions.

[0084] A light scanning apparatus according to claim 31 is characterizedby a liquid crystal element that deflects a light beam from a lightsource to adjust the position of a light spot formed by said light beamon a scanned face; wherein the ratio of a change in transmissivity (%)of said liquid crystal element caused by the deflection to a deflectingangle (minute) is equal to or smaller than 2.0 (%/minute).

[0085] The invention according to claim 32 is characterized in that, inthe light scanning apparatus as claimed in claim 31, said ratio is equalto or smaller than 2.0 (%/minute) in 10 or more ranges of saiddeflecting angle, said ranges appearing cyclically.

[0086] The invention according to claim 33 is further characterized by,in the light scanning apparatus as claimed in claim 31, including adetecting unit that detects intensity of said light beam on said scannedface.

[0087] The invention according to claim 34 is further characterized, inthe light scanning apparatus as claimed in claim 31, by a compensatingunit that compensates intensity of said light beam on said scanned face.

[0088] An image forming apparatus according to claim 35, ischaracterized by: a scanned face; and a light scanning apparatus thatscans said scanned face with a light beam and forms a electrostaticlatent image on said scanned face; wherein said light scanning apparatusfurther comprises a liquid crystal element that deflects said light beamfrom a light source to adjust the position of a light spot formed bysaid light beam on said scanned face; and the ratio of a change intransmissivity (%) of said liquid crystal element caused by thedeflection to a deflecting angle (minute) is equal to or smaller than2.0 (%/minute).

[0089] Other objects, features, and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0090] FIGS. 1A-1C are schematic diagrams showing a light scanningapparatus according to an embodiment of the present invention;

[0091] FIGS. 2A-2D are schematic diagrams for explaining a liquidcrystal deflecting element used as a beam deflecting unit;

[0092] FIGS. 3A-3C are schematic diagrams showing a light intensitycompensating unit according to an embodiment;

[0093]FIGS. 4A and 4B are schematic diagrams for explaining anembodiment according to claim 8;

[0094]FIG. 5 is a schematic diagram for explaining an embodimentaccording to claim 14;

[0095]FIG. 6 is a schematic diagram showing a light scanning apparatusaccording to another embodiment of the present invention;

[0096] FIGS. 7A-7D are schematic diagrams for explaining thecompensation of scan line bend by a liquid crystal element array;

[0097]FIG. 8 is a schematic diagram showing an image forming apparatusaccording to an embodiment of the present invention;

[0098]FIG. 9 is a schematic diagram showing an image forming apparatusaccording to another embodiment;

[0099]FIG. 10 is a exploded perspective view showing the structure of alight source unit provided to the light scanning apparatus according toan embodiment;

[0100]FIG. 11 is a exploded perspective view for explaining an attachingmethod of the light source unit to the side wall of the light scanningapparatus according to an embodiment;

[0101] FIGS. 12A-12D are schematic diagrams for explaining the lightpath deflection of a light beam incident to a liquid crystal element;

[0102]FIG. 13 is an exploded perspective view showing a conventionallight source apparatus that emits four light beams;

[0103]FIG. 14A is an exploded perspective view showing a light sourceapparatus that is constructed by adding a liquid crystal element to thelight source apparatus shown in FIG. 13;

[0104]FIG. 14B is a perspective view showing major optical elements ofthe light source apparatus shown in FIG. 14A;

[0105] FIGS. 15A-15F are perspective views showing various examples ofthe liquid crystal elements to be added to the light source apparatusshown in FIG. 14A;

[0106]FIG. 16 is a schematic diagram showing a light scanning apparatusaccording to an embodiment of the present invention of which opticalelements are expanded on a plane parallel to the deflecting reflectiveface;

[0107]FIG. 17 is a schematic diagram showing an image forming apparatusaccording to an embodiment;

[0108]FIGS. 18A and 18B are graphs illustrating the change intransmissivity of a liquid crystal element accompanying beam deflection;

[0109]FIG. 19 is another graph illustrating the change in transmissivityof a liquid crystal element accompanying beam deflection;

[0110]FIG. 20 is yet another graph illustrating the change intransmissivity of a liquid crystal element accompanying beam deflection;

[0111]FIG. 21 is a schematic diagram showing an image forming apparatusaccording to another embodiment of the present invention; and

[0112]FIG. 22 is a schematic diagram showing an image forming apparatusaccording to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0113] Preferred embodiments of the present invention will be describedby reference to the drawings.

[0114]FIG. 1A is a schematic diagram showing a multi-beam light scanningapparatus according to an embodiment of the present invention.

[0115] Semiconductor laser diodes 11 a and 11 b are provided as lightsources, and each of them emits a diverging light beam. The light beamsare transformed into parallel beams by coupling lenses 12 a and 12 b sothat they fit the remaining optical system in this case. In anothercase, the light beams may be transformed into weakly diverging beams orweakly converging beams.

[0116] The light beams transformed by the coupling lenses 12 a and 12 bgo through beam deflecting units 40 a and 40 b, respectively. The lightbeams are converged in the sub-scan directions by a cylindrical lens 13,and form long linear images in the main-scan directions on thedeflecting reflective face of a polygon mirror 14. The linear imageformed by one light beam is separated from the other by a predeterminedsmall distance.

[0117] As the polygon mirror 14 rotates at a constant rotative speed,the light beams are deflected at a constant rotative speed, and passthrough lenses 151 and 152. The light beams are reflected by areflective mirror 153, and form two light spots separated from eachother by a predetermined distance in the sub-scan directions on thephotosensitive face of a drum-shaped photoconductive photosensitive body16. The light beams scan the photosensitive body 16 on the two scanlines simultaneously.

[0118] The lenses 151 and 152 form an “fθ lens”. The fθ lens correspondsto a “scan image forming optical system”, and the lenses 151 and 152correspond to “resin lenses”.

[0119] The deflected light beams, before scanning the photosensitivebody 16, reach a light sensor 19 provided outside the recording region.The light sensor 19 outputs a signal for synchronization. That is, thelight sensor 19 corresponds to “synchronization detecting unit of scan”.

[0120] The light sensor 19 corresponds to “light intensity detectingunit” that detects the light intensity of each light beam. That is, thelight scanning apparatus is provided with the light intensity detectingunit 19 for detecting the light beam intensity. This light intensitydetecting unit also corresponds to the synchronization detecting unit ofscan.

[0121] The light sensor 19 also corresponds to the scan line pitchdetecting unit.

[0122] As shown in FIG. 1C, the sensor unit of the light sensor 19 hastwo PIN photo diodes 191 and 192 of which light reception faces arelinear-shaped long in one direction. The light reception face of the PINphoto diode 191 is parallel to the sub-scan directions, and the lightreception face of the PIN photo diode 192 is tilted to the sub-scandirections. The output of the PIN photo diode 191 is used forsynchronization and light intensity detection.

[0123] The scan line pitch can be adjusted by making each semiconductorlaser emit the light beam separately to detect the difference in time ofthe output of the PIN photo diodes 191 and 192. Since the distancebetween the PIN photo diodes 191 and 192 linearly changes in thesub-scan directions, the position in the sub-scan directions at whichthe light beam crosses the light sensor 19 can be obtained based on thetime difference.

[0124] For example, “t1” and “t1′” indicate theoretical time and actual(measured) time, respectively, required for the light beam emitted bythe semiconductor laser 11 a to cross the PIN photo diodes 191 and 192of the light sensor 19. Likewise, “t2” and “t2′” indicate theoreticaltime and actual (measured) time, respectively, required for the lightbeam emitted by the semiconductor laser 11 b to cross the PIN photodiodes 191 and 192 of the light sensor 19. Time t1′−t1 and t2′−t2 aredisplacements of the scan line of each light beam in the sub-scandirections.

[0125] Information of the displacement of the scan line is obtained by acontrol unit 10 shown in FIG. 1B. The control unit 10 is configured by aCPU or a microcomputer, for example. The control unit 10 may be a partof a control unit that controls the entire light scanning apparatus orthe entire image forming apparatus using the light scanning apparatus.

[0126] The control unit 10 calculates the time Δt1=t1′−t1 andΔt2=t2′−t2. A compensative amount of the scan line position of the lightbeam emitted by the semiconductor laser 11 a based on the calculation ofΔt1 is calculated, and a compensative amount of the scan line positionof the light beam emitted by the semiconductor laser 11 b based on thecalculation of Δt2 is calculated.

[0127] The control unit 10 controls beam deflecting units 40 a and 40 bbased on the (respective) compensative amounts to bend each light beamin the sub-scan directions so as to obtain an appropriate scan linepitch on the scanned face. In an embodiment shown in FIG. 1A, the beamdeflecting unit 40 a and 40 b are liquid crystal deflecting elements,for example. The liquid crystal deflecting element is described indetail below.

[0128] A deflected angle φ_(z) of the light beam in the sub-scandirections and a displacement amount ΔZ of the light spot in thesub-scan directions (in this case, a displacement amount of the scanline itself in the sub-scan directions) satisfies the followingrelation:

ΔZ=f _(cp) *m _(z)*tan φ_(z), or

φ_(z)=tan⁻¹ [ΔZ/(f _(cp) *m _(z))]

[0129] where m_(z) is widthwize magnification in the sub-scan directionsof the entire optical system between the light source and the scannedface, and f_(cp) is the focal distance of the coupling lenses 12 a and12 b.

[0130] In the case that the light intensity of the light beam changes asit is deflected by the beam deflecting unit in the sub-scan directions,the control unit 10 controls driver circuits 11 a 1 and 11 b 1 drivingthe semiconductor lasers 11 a and 11 b so that the light intensities ofthe light spots are substantially equal on the scanned face.

[0131] In the light scanning apparatus shown in FIG. 1A, the light beamsemitted by the light sources 11 a and 11 b are deflected by the lightdeflecting unit 14, and the deflected light beams are converged on thescanned face 16 by the scan convergence optical system 151, 152, and153. The light scanning apparatus is further provided with light spotposition adjusting units 40 a and 40 b that adjust the light spotposition on the scanned face, and light intensity compensation units 11a 1 and 11 b 1 that compensate change in the light intensity of thelight spots accompanying the adjustment of the light spot position bythe light spot position adjusting unit.

[0132] The light scan uses multiple light beams emitted by “N” (=2)light sources 11 a and 11 b (multi-beam scan method). The light spotposition adjusting unit has “N” (=2) beam deflecting units 40 a and 40 bthat bend light beams from “N” (=2) light sources to the lightdeflecting unit in the sub-scan directions, the beam deflecting units 40a and 40 b being provided between the light sources 11 a, 11 b and thelight deflecting unit 14. The beam deflecting units 40 a and 40 b adjustthe scan line pitch of the multi-beam scan method. The beam deflectingunits 40 a and 40 b are “liquid crystal deflecting elements”.

[0133] The light scanning apparatus is further provided with the lightintensity detecting unit 19 that detects intensity of the light beam.The light intensity detecting unit 19 concurrently functions as asynchronization detecting unit of the light scan. The light intensitycompensating units 11 a 1 and 11 b 1 are the units for adjustingemission intensity of the light sources 11 a and 11 b, respectively.Resin lenses 151 and 152 are provided in the light path from the lightsource to the scanned face.

[0134] A “liquid crystal deflecting element” is briefly described thatis the beam deflecting units 40 a and 40 b constructing the light spotposition adjusting unit shown in FIG. 1A. The liquid crystal deflectingelement is an optical element that bends the direction of light beamusing the function of liquid crystal. Various types of liquid crystaldeflecting elements are known.

[0135] Liquid crystal deflecting elements can be driven by an electronicsignal or a magnetic signal. The liquid crystal deflecting element to beused as the beam deflecting unit may be driven either electrically ormagnetically. In the following description, the beam deflecting unit ofFIG. 1 is assumed to be an electrically driven liquid crystal deflectingelement.

[0136] The electrically driven liquid crystal deflecting elements areroughly divided into the liquid crystal deflecting elements of whichrefractive index is changed and the liquid crystal deflecting elementsthat induce diffraction. The liquid crystal deflecting elements of whichrefractive index is changed are described below.

[0137] Such a liquid crystal deflecting element is described in JapanesePatent Laid-open Application No. 63-240533. FIG. 2 shows such anexample.

[0138] In FIG. 2B, liquid crystal 1 is nematic liquid crystal withpositive dielectric anisotropy that is hermetically sealed in a thinlayer between a pair of transparent orientation films 2A and 2B betweenwhich a gap is maintained at a predetermined width by spacers 3. Theliquid crystal molecule designated by a numeral 1A is long in thedirection of molecular axis. The orientation film 2A is orientationprocessed so that the molecular axis of the liquid crystal molecule 1Abecomes perpendicular to the face of the orientation film. Theorientation film 2B is orientation processed so that the molecular axisof the liquid crystal molecule 1A becomes parallel to the face of theorientation film.

[0139] A transparent electric resistance film 4 made of ZnO, forexample, is formed on the opposite side of the orientation film 2A,which side is not in contact with spacers 3.

[0140] The transparent electric resistance film 4, the orientation films2A, 2B, and the liquid crystal 1 are sandwiched by a pair of transparentglass substrates 5A and 5B. A transparent electrode film 6 made of ITO,for example, is formed over the entire face of the glass substrate 5B atthe orientation film 2B side.

[0141] On the other hand, electrodes 7A and 7B of which patterns areshown in FIG. 2A are formed on the face of the glass substrate 5A at theorientation film 2A side. These electrodes 7A and 7B touch the electricresistance film 4 as shown in FIG. 2B.

[0142] The electrodes 7A and 7B need to be formed by ITO, for example,if the electrodes overlap a region through which the light beamtransmits. The electrodes 7A and 7B, however, may be formed bynontransparent metal film, for example, if the electrodes do not overlapthe region through which the light beam transmits (if the electrodes 7Aand 7B do not block the light beam). The electrodes 7A and 7B shown inFIG. 2 are transparent electrodes.

[0143] If the electrode film 6 and the electrode 7B are grounded, and avoltage V is applied between the terminals A and B of the electrodes 7Aand 7B as shown in FIG. 2A, the potential of the electric resistancefilm 4 linearly decreases from the electrode 7A to the electrode 7B.Accordingly, there exists an electric field (that is directed in thehorizontal directions) between the electric resistance film 4 and thetransparent electrode film 6 linearly decreasing from the upper side tothe lower side of FIG. 2B.

[0144] This electric field affects the liquid crystal 1 and causes theliquid crystal molecule 1A to turn so that the molecular axis of theliquid crystal molecule 1A becomes parallel to the electric field. Therotative angle of the liquid crystal molecule 1A is linearlyproportional to the strength of the electric field. The molecular axisof the liquid crystal molecule 1A becomes closer to the direction of theelectric field at the electrode 7A side, while the molecular axis of theliquid crystal molecule 1A remains substantially parallel to theelectrode film 6 at the electrode 7B side since the electric fieldstrength is substantially zero.

[0145] The permittivity of the liquid crystal molecule 1A is great inthe directions parallel to its molecular axis and small in thedirections perpendicular to its molecular axis. Accordingly, therefractive index becomes greater in the direction parallel to themolecular axis. Since the molecular axis of the liquid crystal molecule1A distributes as described above, the refractive index of the liquidcrystal 1 becomes high at the electrode 7A side where the molecular axisis substantially parallel to the electric field, and low at theelectrode 7B side where the molecular axis is substantiallyperpendicular to the electric field. The refractive index linearlydecreases from the electrode 7A side to the electrode 7B side.

[0146] Accordingly, when the refractive index of the liquid crystaldeflecting element is distributed as described above, a light beamincident on the liquid crystal deflecting element from the left side ofFIG. 2B is deflected towards the high refractive index side (the upperside of FIG. 2B) due to the distribution of the refractive index.

[0147] When the electrode 7A, instead of 7B, is grounded, and anopposite voltage is applied between terminals A and B, the refractiveindex decreases from the electrode 7B side to the electrode 7A side, andthe transmitting light beam is deflected downwards in FIG. 2B due to thedistribution of the refractive index.

[0148] The liquid crystal deflecting element bends a light beam usingthe distribution of the refractive index as described above.

[0149] The amount of deflecting, that is, a “deflecting angle”,saturates at an intrinsic value for each liquid crystal deflectingelement. The deflecting angle does not exceed the saturation value. Theliquid crystal element may be driven with a direct voltage signal.However, a pulse signal or a sine wave signal of which average voltageis near zero is preferred so as to extend the life of the liquid crystaldeflecting element.

[0150] In the case that the pulse signal is applied to the liquidcrystal deflecting element, the deflecting angle depends on the dutyratio of the pulse signal as well as the voltage V between the terminalsA and B.

[0151] In the case of the nematic type liquid crystal deflecting elementdescribed above, the light beam is deflected by changing the drivingvoltage applied to the liquid crystal 1 so as to change the anisotropyin refractive index Δn (=ne−no) of a regular light beam and an irregularlight beam and consequently change the refractive index. Thetransmissivity cyclically changes depending on the deflecting angle ofthe light beam due to the anisotropy as shown in FIG. 2D.

[0152] The transmissivity T of the light beam transmitting through theliquid crystal 1 is:

T=1−[sin² [(π/2)(1+u ²)^(1/2)]]/(1+u ²), and

u=2*Δn*d*λ

[0153] with d indicating the thickness of the liquid crystal 1, λ beingthe wavelength of the light beam, and Δn indicating the aboveanisotropy. Strictly speaking, the x-axis of a graph shown in FIG. 2Dindicates the above variable “u”.

[0154] In the case that the liquid crystal deflecting elements are usedas the beam deflecting units 40 a and 40 b, the control unit 10determines a deflecting angle by which the light beam is to be deflectedand controls the driving voltage to be applied to the beam deflectingunits 40 a and 40 b. Because the transmissivity of the light beam thattransmits through the beam deflecting units 40 a and 40 b can beobtained based on the above theoretical expression, one can prepare atable for obtaining the compensatory amount of the light intensity basedon the deflecting amount of the light beam. The control unit 10 candetermine the compensatory amount based on the above table and controlsthe emission intensity of the semiconductor lasers 10 a and 11 b withthe driver circuits 11 a 1 and 11 b 1. In this case, in theory the lightsensor 19 does not need to detect the light intensity of the light spot.

[0155] The actual transmissivity of the liquid crystal may vary due todispersion or degrading over time of parts and assembly forming theactual light scanning apparatus. Keeping this problem in mind, the lightintensity of the light spot needs to be monitored by the light sensor 19so as to compensate the light intensity.

[0156] It is not necessary to adjust the position of each scan line.Only the relative position of one scan line to the other scan line needsto be adjusted by using one beam deflecting unit either 40 a or 40 b(“N−1” beam deflecting units in the case of “N” light sources). In thiscase, the light intensity of only one semiconductor laser needs to beadjusted.

[0157] The case in which the liquid crystal deflecting element is usedas the light spot position adjusting unit is described above. The lightspot position adjusting unit is not limited to the liquid crystaldeflecting element and variations may be made.

[0158] For example, a cylindrical lens may be provided for each lightbeam that converges the light beam into a linear image on the deflectingreflective face position of the polygon mirror, and is rotated around anaxis parallel to the main-scan directions. The cylindrical lens may beshifted in the sub-scan directions. An electro-optical element oracoustic-optical element (AOM) may be used instead of the cylindricallens. A transparent parallel plate may be provided between thesemiconductor laser and the coupling lens and be rotated around an axisparallel to the main scan directions.

[0159] Although not shown in FIG. 1, an aperture for beam shaping isgenerally used in the light scanning apparatus. In the case that thisaperture is provided between the semiconductor laser and the beamdeflecting unit, the deflecting of the light beam in the sub-scandirections does not affect the blocking of the light beam by thisaperture.

[0160] In the case that the aperture is provided between the beamdeflecting unit and the polygon mirror, the light beam moves in thesub-scan directions at the aperture due to the deflecting by the beamdeflecting unit, and the amount of the light beam blocked by theaperture changes as the light beam is deflected.

[0161] The above change in the light intensity of the light spot may becompensated by controlling the emission intensity of the semiconductorlaser. However, since the function of the aperture for beam shaping isto form a light spot of a desired shape on the scanned face, a morepreferable method to solve this problem may be to provide a unit forshifting the aperture for beam shaping in the sub-scan direction so asto shift the aperture in the sub-scan direction, and to cause thedeflected light beam to always pass through the center of the aperture.

[0162] The compensation of the light intensity of the light spot due tothe disposition of the aperture is described below with reference toFIG. 3.

[0163] FIGS. 3A-3C show an aperture for beam shaping 27 provided betweenthe beam deflecting unit 40 and the polygon mirror 14. Although thecylindrical lens 13 is not shown, the aperture 27 is provided at thecylindrical lens 13 side of the beam deflecting units 40 a and 40 b.Reference numerals are generalized. For example, the reference numerals11, 12, and 40 denote reference numerals 11 a, 12 a, 40 a or 11 b, 12 b,40 b.

[0164]FIG. 3A shows a state in which the beam deflecting unit 40 doesnot bend the light beam. The emission source of the semiconductor laser11 and the center of the aperture 27 are on the light axis of thecoupling lens 12. Accordingly, the principal ray of the light beamemitted by the semiconductor laser 11 matches the light axis of thecoupling lens 12 and goes through the center of the aperture 27.

[0165]FIG. 3B shows a state in which the beam deflecting unit 40 bendsthe light beam in the sub-scan directions (the vertical directions ofFIG. 3B). In this case, if the aperture 27 remains at the same positionas FIG. 3A, a portion of the deflected light beam (of which powerdistribution is Gaussian symmetric to the optical axis ray) is blockedby the aperture 27. As a result, the amount of light transmitted to thepolygon mirror side decreases, and the light intensity of the light spotdecreases.

[0166] To solve this problem, when the beam deflecting unit 40 bends thelight beam, the aperture 27 is shifted by a distance A depending on thebeam deflecting angle θ so that the principal ray of the deflected lightbeam goes through the center of the aperture 27. The shifted distance Δof the aperture 27 can be obtained by Δ=L*tan θ, where “L” is thedistance between the beam deflecting unit 40 and the aperture 27.

[0167] The change in the light intensity of the light spot caused by theaperture 27 blocking the light beam can be compensated by shifting theaperture 27 as described above. There still remains the change in thelight intensity of the light spot caused by the beam deflecting unit 40of which transmissivity changes, and this change can be compensated byadjusting the emission intensity of the semiconductor laser 11 asdescribed above.

[0168] Another example of the beam deflecting unit (the light spotposition adjusting unit) is described with reference to FIGS. 4A and 4B.According to this example, a semiconductor laser, a coupling lens, andan aperture for beam shaping are integrated with a holder. The apertureis positioned so that a light beam emitted by the semiconductor laserthat goes through the center of the coupling lens further goes throughthe center of the aperture.

[0169] As shown in FIGS. 4A and 4B, the semiconductor laser 11, thecoupling lens 12, and the aperture 27 for beam shaping are integrated bya holder HL that fits with a housing HOU in which the other opticalsystem is provided.

[0170] The semiconductor laser 11, the coupling lens 12, and theaperture 27 integrated by the holder HL are mutually positioned asdescribed below.

[0171] The emission source of the semiconductor laser 11 is eccentric bya predetermined small distance (to the direction perpendicular to FIG.4A) from the optical axis of the coupling lens 12. The semiconductorlaser 11 is press fitted to the holder HL. The coupling lens 12 isattached after the relative position of the coupling lens 12 to thesemiconductor laser 11 is adjusted.

[0172] Because the emission source of the semiconductor laser 11 iseccentric (offset) from the optical axis of the coupling lens 12, a raytransmitting through the center of the coupling lens 12 among the lightbeams emitted by the semiconductor laser 11 and passing through thecoupling lens is tilted relative to the optical axis of the couplinglens 12.

[0173] The aperture 27 is positioned so that the light beam emitted bythe semiconductor laser 11 and passing through the center of thecoupling lens 12 goes through the center of the aperture 27.

[0174] The holder HL fitted to the housing HOU is rotatable on an axisparallel to the optical axis of the coupling lens 12 and going throughthe emission source of the semiconductor laser 11. FIG. 4B shows a statein which the holder HL is rotated. When the holder HL is rotated, thedirections of the ray (shown by a line) passing through the center ofthe coupling lens 12 changes around the above rotative axis like aprecession. The light beam is deflected by this change in direction.

[0175] The positional relationship among the semiconductor laser 11, thecoupling lens 12, and the aperture 27 is fixed by the holder HL. Even ifthe light beam is deflected, the ray passing through the center of thecoupling lens of the deflected light beam always goes through the centerof the aperture 27. The amount of the deflected light beam blocked bythe aperture 27 does not change.

[0176] The aperture 27 is displaced keeping the same positionalrelationship among the aperture, the semiconductor laser, and thecoupling lens. This construction corresponds to the light intensitycompensating unit.

[0177] The aperture 27 is integrated with the semiconductor laser 11 andthe coupling lens 12 by the holder in the embodiment shown in FIG. 4,but the aperture 27 may be separated. In this case, the aperture 27 isdisplaced following a circular displacement trace with anotherdisplacement mechanism so that the ray passing through the center of thecoupling lens goes through the center of the aperture.

[0178] In the case of the embodiment shown in FIG. 4, since the lightbeam is deflected around the rotational axis like a precession as theholder HL rotates, the light spot position displaces in the main scandirections as well as the sub-scan directions. In practice, thedisplacement in the main scan directions is so small that the scan linepitch can be compensated by compensating the displacement component inthe sub-scan directions.

[0179] The effect of the displacement of the light spot in the main scandirections is removable by detecting synchronization. In the case thatthe displacement to the main scan directions effects the compensation ofthe scan line pitch, the above liquid crystal deflecting element, forexample, is provided to adjust the light spot position in the main scandirections and to compensate the displacement of the light spot in themain scan directions due to the rotation of the holder.

[0180] The light intensity compensating unit that controls the emissionintensity of the semiconductor laser, that displaces the aperture, andthat integrates the aperture with the rotation of the semiconductorlaser and the coupling lens, for example, are described above. The lightintensity compensating unit is not limited to these examples andvariations may be made. For example, a light intensity compensating unitin which a transmissivity adjusting unit is provided between the lightsource and the light deflecting unit can be provided.

[0181]FIG. 5 shows the case in which the above light intensitycompensating unit is applied to the embodiment shown in FIG. 1. Theconstruction beyond the polygon mirror is the same as FIG. 1A.

[0182] As shown in FIG. 5, transmissivity adjusting units 43 a and 43 b(light intensity compensating unit) are provided in the light path ofthe light beam transmitted by the beam deflecting units 40 a and 40 b.

[0183] The transmissivity adjusting units 43 a and 43 b make the lightintensities of the light spots on the scanned face substantially equalby adjusting the transmissivity of the light beams. There are varioustransmissivity adjusting units.

[0184] The light beams emitted by the semiconductor lasers 11 a and 11 bare substantially linearly polarized. The transmissivity of the lightbeam can be adjusted by rotating a polarizer (used as the transmissivityadjusting units 43 a and 43 b) with a driving unit (not shown) of whichdriving amount is determined by the control unit.

[0185] A rotative gradation ND filter of which transmissivity changeswith gradation as it is rotated may be used as the transmissivityadjusting units 43 a and 43 b. The transmissivity may be adjusted byrotating the rotative gradation ND filter with a not shown driving unit.

[0186] If the light intensities of light spots need to be mutuallyequal, only one transmissivity adjusting unit may suffice.

[0187] There are liquid crystal deflecting elements that use diffractioninstead of the change in refractive index to bend the light beam. In thecase that such a liquid crystal deflecting element is used as a beamdeflecting unit, the transmissivity can be adjusted by changing thepitch of diffraction lattice and, consequently, the diffractionefficiency.

[0188] In this case, a transmissivity adjusting unit 43 a or 43 b may beprovided on at least one of the two light paths.

[0189] An example in which beam deflecting units such as the liquidcrystal deflecting elements are provided between the light source andthe light deflecting unit is described above. A case is described belowin which a liquid crystal deflecting element array of a plurality ofliquid crystal deflecting elements arranged in the main scan directions,each liquid crystal deflecting element having a function to bend thelight beam in the sub scan directions, is provided between the lightdeflecting unit and the scanned face as the light spot positionadjusting unit or a portion thereof.

[0190] In the case that the beam deflecting unit is provided between thelight source and the light deflecting unit, the compensation of scanline pitch is possible, but the compensation of a deflected scan line isdifficult. However, in the case that the liquid crystal deflectingelement array is provided between the light deflecting unit and thescanned face, the compensation of the deflected scan line (including thetilt scan line) is possible, but the compensation of the scan line pitchis difficult.

[0191] Accordingly, the method that uses only the liquid crystaldeflecting element array as the light spot position adjusting unit isappropriate for the case of a tandem type image forming apparatus inwhich each photosensitive body is scanned with the single beam scanmethod or each photo sensitive body is scanned with the multi-beam scanmethod using multiple light beams from a semiconductor laser array (thechange in scan line pitch can be compensated by rotative adjustment ofthe light source in this case).

[0192]FIG. 6 is a schematic diagram showing an embodiment of a lightscanning apparatus 20. The components that are shown above in FIG. 1 arereferred to by the same reference numerals to make the followingdescription simple in principle.

[0193] A reference numeral 60 denotes a light source unit. The lightsource unit includes a semiconductor laser, a coupling lens, and anaperture for beam shaping. The light source unit emits a parallel andshaped light beam.

[0194] The light beam emitted by the light source unit 60 passes througha beam deflecting unit 40 (identical to the beam deflecting units 40 aand 40 b described with reference to FIG. 1), and converged by acylindrical lens 13 into a long linear image in the main scan directionson a deflecting reflective face position of the polygon mirror 14. Thelight beam is deflected as the polygon mirror 14 rotates, and forms alight spot on a drum-shaped photosensitive body 16, which is anembodiment of the scanned face, for performing a single beam scan.

[0195] A light sensor 19 is the same as the light sensor 19 showed inFIG. 1B. The light sensor 19 detects the light intensity of the lightspot and the position of the scan line in the sub-scan directions (to beobtained by detecting the difference in time at which two PIN photodiodes detect the scan line). The light sensor 19 also providessynchronization to start the scan.

[0196] The light beam passes through a scan convergence optical system15A and a liquid crystal deflecting element array 40A. Although notshown in FIG. 6, a fraction (detection light beam) of the light beampassing through the liquid crystal element array 40A is separated by ahalf mirror, for example, from the light path to the photo sensitivebody, and is led to a scan line bend detecting unit (not shown).

[0197] In this embodiment, the scan line position in the sub scandirections is compensated by deflecting the light beam in the sub scandirections by the beam deflecting unit 40, and the scan line bendincluding tilt is compensated by the liquid crystal deflecting elementarray 40A.

[0198] Light spot position adjustment by the liquid crystal deflectingelement array 40A is described with reference to FIG. 7.

[0199] In FIG. 7A, the horizontal directions are the main scandirections. Reference numerals Li (i=1-10) denote electrically drivenliquid crystal deflecting elements. That is, 10 liquid crystaldeflecting elements L1-L10 are closely and continuously provided in themain scan directions in this embodiment. Each liquid crystal deflectingelement Li of this embodiment is of the same size and of the same pitch.Each liquid crystal deflecting element Li may be of the same type asdescribed above with reference to FIG. 2.

[0200] Reference numerals Di (i=1-10) denote driver circuits that drivethe liquid crystal deflecting elements Li. The driver circuits Di arecontrolled by a controller 22. The controller 22 may be the control unit10 shown in FIG. 1. The deflecting direction of the liquid crystaldeflecting elements Li is set in the sub scan directions.

[0201] It is noted that, although each liquid crystal deflecting elementLi is independently driven by the corresponding driver circuit Di,liquid crystal, orientation films sandwiching the liquid crystal, andtransparent electrodes are common for all liquid crystal deflectingelements Li. Only the electrodes to which driving voltages are appliedand transparent resistance films connecting the transparent electrodesare separated by a liquid crystal deflecting element Li (i=1-10).

[0202]FIG. 7B shows an embodiment of the scan line bend detecting unitdescribed above. As described above, the fraction (detection light beam)of the light beam that has passed through the liquid crystal deflectingelement array 40A is separated from the light path to the photosensitive body 16 and is led to a scan line bend detecting unit 23.

[0203] Although not shown in FIG. 6, the scan line bend detecting unitis positioned at an optically equivalent position to the photosensitivebody 60 in connection with the detection light beam. Accordingly, thedetection light beam is converged into a light spot on the photoreception face of the scan line bend detecting unit.

[0204] The scan line bend detecting unit 23 shown in FIG. 7B is providedwith light reception faces of area sensors P1-P10 of which quantity isthe same as that of the liquid crystal deflecting elements Li, the lightreception faces being arrayed in the main scan directions. The lightreception face of each area sensor Pi is disposed at an opticallyequivalent position to the scanned face (the photosensitive face of thephotosensitive body 16). The detection light beam separated from thedeflected light beam scans the light reception faces.

[0205] The light reception faces of the area sensors Pi correspond tothe liquid crystal deflecting elements Li of the liquid crystaldeflecting element array 40A. The light reception face of the areasensor Pi and the corresponding liquid crystal deflecting element Li aremutually positioned so that the deflected light beam theoreticallypasses through the center of the liquid crystal deflecting element Liand converges into the light spot at the center of the area sensor Pi.

[0206] The area sensors Pi are fixed on a fixing plate 23S. The fixingplate 23S is made of materials of which thermal coefficient of expansionis 1.0×10⁻⁵/° C. or less, such as glass (thermal coefficient ofexpansion: 0.5×10⁻⁵/° C.), ceramic material (thermal coefficient ofexpansion: 0.7×10⁻⁵/° C.), and silicon carbide (for example, alumina;thermal coefficient of expansion: 0.4×10⁻⁵/° C.) so that the fixingplate 23S is not substantially affected by the change in temperature.Otherwise, the detection by the area sensors Pi becomes inaccurate dueto the disposition of the light reception face and the change in mutualpositional relationship between the area sensors Pi and thecorresponding liquid crystal deflecting elements Li.

[0207] Additionally, the fixing plate 23S is desired to be made ofnonconductive material as described above to avoid electric noise beingcaused among the area sensors Pi. A region RY shown in FIG. 7B is aregion corresponding to the effective writing width of the scanned face.

[0208] A case is described below in which the scan line bend iscompensated by the liquid crystal deflecting element array 40A.

[0209] For example, before an image forming process by optical scanning,the polygon mirror 14 is rotated and the light source unit 60 isactivated. The light source is intermittently caused to emit the lightbeam so that the detection light beam of each emission comes to eacharea sensor P1-P10 of the scan line bend detecting unit 23. The scanline bend detecting unit 23 outputs the position of the light spot inthe sub scan directions detected by the area sensors Pi (i=1-10).

[0210] In FIG. 7C, ten black dots indicate detected positions of thelight spots. The dotted line indicates an ideal scan line that is linearin the main scan directions. The controller 22 approximates the actualscan line including the ten light spot positions in the sub scandirections with a polynomial using the method of least squares, forexample. The polynomial is a detected scan line bend, which is shown bythe solid line in FIG. 7C.

[0211] The controller 22 calculates the deflecting direction and thedeflecting angle in the sub scan directions at the liquid crystaldeflecting element Li of the sub scan liquid crystal deflecting elementarray 40A in order to compensate such a scan line bend. In FIG. 7C,regions Si (i=1-10) indicate regions (allocated compensation regions) inwhich the liquid crystal deflecting element Li of the sub-scan liquidcrystal deflecting element array need to bend the light beam, and upwardor downward arrows in the regions Si indicate the directions ofdeflecting.

[0212] The controller 22 determines a signal with which a liquid crystaldeflecting element Li is to realize the above deflecting direction andthe deflecting angle, and applies the signal to the driver circuit Di(i=1-10). In this example, the deflecting direction is controlled byselecting the polarization of the voltage to be applied to the liquidcrystal deflecting element Li and the electrode to be grounded. Thedeflecting angle is controlled by applying pulse voltages of thatvoltage and adjusting its duty ratio.

[0213] As described above, before the image forming process is started,an adjusted deflecting amount of the liquid crystal deflecting elementLi (i=1-10) of the liquid crystal deflecting element array 40A isrealized. Of course, in the case that the detected scan line bend is sosmall that no compensation is required, the compensation of the scanline bend of the sub scan liquid crystal deflecting element array is notrequired.

[0214]FIG. 7D shows a state of scan line compensated by the sub scanliquid crystal deflecting element array. Yi (i=1-10) denotes a portion(compensation allocated region) in the scan region of the scanned faceto which each liquid crystal deflecting element Li is assigned andresponsible for compensation.

[0215] The scan line indicated in FIG. 7D may appear a little“staggered” since the scan line bend is extremely emphasized in FIG. 7C,but the scan line bend in practice is within the range of 0.1-0.2 mm.Even if one liquid crystal deflecting element Li is assigned to a scanregion of 30 mm, the scan line becomes substantially linear.

[0216] It is noted that, if the number of liquid crystal deflectingelements in the sub scan liquid crystal deflecting element array isfurther increased, and the allocated compensation region of the liquidcrystal deflecting element Li becomes smaller, the compensation of thescan line bend becomes more accurate.

[0217] Especially, if the width of the sub scan liquid crystaldeflecting element Li in the direction of main scan directions in thesub scan liquid crystal element array is small enough (2-5 mm, forexample), the change in deflecting amount between adjacent liquidcrystal deflecting elements becomes substantially equal, andconsequently, the scan line becomes a substantially continuous straightline.

[0218] Those skilled in the art easily recognize that the tilt of thescan line, which is an aspect of the scan line bend, can be compensatedin the same manner.

[0219] As described above, the scan line bend detecting unit detects thescan position of the light spot to identify the scan line bend to becompensated. The adjusted deflecting amount of the liquid crystaldeflecting element Li is set based on the determination. Accordingly,even in the case that the scan line bend changes over time, or the fθlens is constructed by a resin lens and the scan line bend changes dueto environmental change, the change can be appropriately compensated byregularly performing the scan position detection.

[0220] According to an embodiment shown in FIGS. 1, 3, 4, 5, and 6, alight scanning method is provided in which a scanned face is scanned bya light beam by deflecting the light beam emitted by a light sourceusing a light deflecting unit, and forming a light spot by convergingthe emitted light beam on the scanned face. The position of the lightspot on the scanned face is adjustable using a light spot positionadjusting unit, and the change in the light intensity of the light spotcaused by the adjustment of the position of the light spot by the lightspot position adjusting unit is compensated using a light intensitycompensation unit.

[0221] According to an embodiment shown in FIGS. 1, 3, and 4, light spotposition adjusting units 40 a, 40 b, and 40, for example, are providedbetween the light source and the light deflecting unit 14 to adjust thescan line pitch of the multi-beam scan method.

[0222] According to the light scanning apparatus of which an embodimentis shown in FIG. 6, the light spot position adjusting unit 40A isprovided between the light deflecting unit 14 and the scanned face 16 tocompensate the scan line bend.

[0223] An embodiment of a n image forming apparatus is described below.

[0224]FIG. 8 shows an embodiment of the image forming apparatus. Theimage forming apparatus 100 is a monochrome laser printer. Aphotosensitive body 111 provided in the image forming apparatus 100 ismade of photo conductive material. Electrostatic latent images formed bylight scanning are made visible as toner images, and the toner imagesare transferred and fixed.

[0225] The laser printer includes a drum-shaped photo conductivephotosensitive body as the photosensitive body 111. Around thephotosensitive body 111, there are provided a charging roller 112(charging unit), a development apparatus 113, a transfer charger 114,and a cleaning apparatus 115. The charging unit may be a corona chargeror a charging brush instead of the charging roller. The transfer charger114 may be substituted for by a contact-type transferring unit such as atransfer roller.

[0226] A light scanning apparatus 117 is provided between the chargingroller 112 and the development apparatus 113, and exposes thephotosensitive body 111 by light scanning. A reference numeral 116indicates a fixing apparatus, and a reference numeral “S” indicates asheet of transfer paper (record sheet).

[0227] In an image forming process, the photo conductive photosensitivebody 111 is rotated counter-clock wise, and its surface is evenlycharged by the charging roller 112. An electrostatic latent image isformed by the light beam from the light scanning apparatus 117. Theformed electrostatic latent image is a so-called “negative latent image”of which an image portion is exposed to the light beam. Thiselectrostatic latent image is reverse developed by the developingapparatus 113, and the toner image is formed on the photosensitive body111.

[0228] When the toner image on the photosensitive body 111 moves to atransfer position, the transfer sheet S is sent to a transfer unit (notshowed). The toner image is superposed and transferred to the transfersheet S by the transfer charger 114. The transfer sheet S on which thetoner image is transferred is sent to the fixing apparatus 116. Afterthe toner image is fixed by the fixing apparatus 116, the transfer sheetS is discharged out of the image forming apparatus. After the tonerimage is transferred, the surface of the photosensitive body 111 iscleaned by the cleaning apparatus 115 so as to remove remaining tonerand paper powder, for example.

[0229] The light scanning apparatus 117 may be the light scanningapparatus with the multi-beam scanning method described with referenceto FIG. 1, for example. In this case, the change in time in the scanline pitch, for example, can be appropriately compensated, and the lightintensity of the light spot of each scan line can be kept substantiallyat the same level, which results in preferable image forming.

[0230] That is, the image forming apparatus shown in FIG. 8 in which thelight scanning apparatus of FIG. 1 is used is an embodiment of the imageforming apparatus according to claim 4 that writes an image on thephotosensitive body 111. The photosensitive medium 111 is aphotoconductive photosensitive body, and electrostatic latent imagesformed by the light scanning is are made visible as toner images.

[0231]FIG. 9 shows a tandem type color image forming apparatus accordingto another embodiment of the present invention. A portion of FIG. 8referred to by the reference numeral 100 (the portion surrounded by adotted line) is called as an “image forming unit”. The tandem type colorimage forming apparatus shown in FIG. 9 is provided with four imageforming units 100Y, 100M, 100C, and 100K arrayed along the path of thetransfer sheet S (toner image transfer medium).

[0232] Each image forming unit 100Y-100K is identically structured tothe image forming unit 100 shown in FIG. 8. The color of toner used ineach image forming unit is different. That is, the image forming unit100Y develops with yellow toner, and the image forming unit 100M, 100C,and 100K develop with magenta, cyan, and black toner, respectively.

[0233] The light scanning apparatus used in each image forming unit maybe that described above with reference to FIG. 6. In these image formingunits, each photosensitive body is scanned with a single beam, but thescan line position in the sub scan directions and the scan line bend arecompensated by an image forming unit. Accordingly, the positionalrelationship between the scan lines of the image forming unit and thescan line bend can be compensated, which results in preferable colorimages without color distortion and color phase change.

[0234] That is, the image forming unit 100Y forms an electrostaticlatent image of yellow component on the photosensitive body and makesvisible the electrostatic latent image with yellow toner into a yellowtoner image. Likewise, the image forming units 100M, 100C, and 100K formelectrostatic latent images of magenta, cyan, and black components,respectively, on the photosensitive body and make visible theelectrostatic latent image with corresponding color toners.

[0235] The transfer paper S is carried from the right side of thedrawing to the left side by a carrier belt 90. While the transfer paperS is carried, the yellow toner image, the magenta toner image, the cyantoner image, and the black toner image are transferred on the transferpaper S. The toner images are superposed on the transfer paper S to forma multi-color image, and fixed by the fixing apparatus 116.

[0236] That is, the image forming apparatus shown in FIG. 9 is providedwith one or more drum-shaped photosensitive bodies arrayed along thepath of toner image transfer medium S. The toner image formed on eachphotosensitive body is transferred on the common recording sheet S toobtain the multi-color image (tandem type image forming apparatus). Anembodiment of the light scanning apparatus according to claim 6 is usedin this image forming apparatus (claim 16). Four photosensitive bodiesare provided, and color images can be formed using the toners, magenta,cyan, yellow, and black (claim 17).

[0237] In the above description of the embodiment of the photo scanningapparatus of the multi-beam scanning method, the case of a dual scanline simultaneous scanning method that uses light beams from twosemiconductor lasers is described. The present invention, however, isnot limited to these embodiments. The present invention is alsoapplicable to such a case that three or more scan lines aresimultaneously scanned using three or more pairs of semiconductor lasersand coupling lenses.

[0238] As described above, the present invention provides a novel anduseful light scanning method, light scanning apparatus, and imageforming apparatus. According to the light scanning method and apparatus,the scan line pitch and the scan line bend can be adjusted. The changein the light intensity of the light spot due to the adjustment can beeffectively compensated. Accordingly, even in the case of a multi-beamscanning method, the inequality of light intensity between scan linescan be effectively avoided. Likewise, even in the case of a tandem typeimage forming apparatus, the inequality of light intensity betweenphotosensitive bodies can be effectively avoided. As a result, the imageforming apparatus using such light scanning apparatus can formpreferable images.

[0239] A light scanning apparatus and an image forming apparatusaccording to another embodiment of the present invention are furtherdescribed with reference to FIG. 1A.

[0240] The light scanning apparatus is an apparatus that scans a scannedface with a light beam (laser beam) emitted by a light source, andincludes a light source unit 18 provided with a light source, acylindrical lens 13, a polygon mirror 14 that functions as a deflectingunit, a scanning optical system 15 including two plastic scan lenses andone reflecting mirror 153, and a photosensitive drum 16 of which surfaceis the scanned face.

[0241]FIG. 1A illustrates a two-beam light scanning apparatus that scanswith two light beams simultaneously. The present invention is alsoapplicable to light scanning apparatuses that scan with more light beams(multi-beam scanning apparatus).

[0242]FIG. 10 is a schematic diagram showing the structure of the lightsource unit 18 shown in FIG. 1A. The light source unit 18 includessemiconductor lasers 211 a and 211 b and corresponding coupling lenses212 a and 212 b fixed on a common base member 243. The semiconductorlasers 211 a and 211 b are press fit in the base member 243. On theother hand, the coupling lenses 212 a and 212 b may be attached to thebase member 243. The position at which the coupling lens is fixed isadjusted depending on the property of the light beam transmittingthrough the coupling lens, that is, the parallelity and the outgoingoptical axis directions.

[0243] In FIG. 1A, the light source unit 18 is structured so that twolight beams 21 a and 21 b cross each other near the deflectingreflective face of the polygon mirror 14. Thanks to such a structure, itis possible to avoid a difference between the optical properties of thetwo light beams caused by the difference in the reflective positions ofthe polygon mirror 14, such as the position of convergence and themagnification. The main scan directions are the directions in which thebeam spot scans the scanned face, and the sub scan directions are thedirections perpendicular to the main scan directions.

[0244] Light beams emitted by the semiconductor lasers 11 a and 11 b areconverted into parallel light beams 21 a and 21 b by the coupling lenses40 a and 40 b, respectively. The two light beams 21 a and 21 b areconverged in the sub scan directions into a long linear image in themain scan directions, and scan the scanned face of the photosensitivebody drum 1 through a scanning optical system (scanning lens) 15.

[0245] The light scanning apparatus (multi-beam scanning apparatus) isoften provided with a light beam position compensating unit thatinitially adjusts the scan line position on the scanned face (light beamposition) and the beam spot pitch, and compensates changes caused byenvironmental reasons and/or elapsed time. The construction of amechanical light beam position compensating unit is described below.

[0246]FIG. 6 is a schematic diagram showing the optical disposition of alight scanning apparatus 20 stored in an optical housing 53, which issectioned by a rotative plane of the polygon mirror. FIG. 11 is aschematic diagram for explaining a method of fixing the light sourceunit 18 to a side wall 54 (254) of the optical housing 53. As shown inFIG. 11, the light source unit 18 is fixed with a pair of screws 245 tothe side wall 254 on which screw holes 255 are provided. At that time,the circular protruding unit 18 a of the light source unit 18 is rotatedin a circular fixing hole 254 a provided on the side wall 254 in therotative directions indicated by “γ” (γ rotation). The rotativedirections “γ” are the rotative directions around the outgoing opticalaxis. Since, as described above, the two light beams 21 a and 21 b arenot parallel, when the light source unit 18 is “γ” rotated, the relativeoptical axis directions of the two light beams in the sub scandirections change. As a result, the light beam position and the lightbeam spot pitch on the scanned face can be compensated.

[0247] However, if the light source unit 18 is attached to the side wall254 of the optical housing 53 with the screws 245 after the light sourceunit 18 is “γ” rotated, the adjustment achieved by the “γ” rotation isoften lost due to a torque applied when the screws are tightened.Conventionally, a process of the adjustment by “γ” rotation and thetightening of the screws needs to be repeated several times on a trialand error basis. If time allocated to the process is limited, a desiredlevel of the adjustment may not be achieved.

[0248] Accordingly, in the case of the light scanning apparatusaccording to the present invention, the light beam position, that is, ascanning position, is compensated by an electrically driven liquidcrystal element 40 besides the mechanical light beam positioncompensating unit. The liquid crystal element 40 can deflect the lightbeam by a small angle (several mrad, for example). If the liquid crystalelement 40 is provided at any position between the coupling lenses 12 aand 12 b and the cylindrical lens 13 shown in FIGS. 1A and 6, the degreeof freedom in mechanical layout design in the optical housing 53 can beincreased.

[0249] Where the focal distance of the coupling lens is Fcol [mm]; thesub-scan magnification of the entire optical system is m [times]; thelight deflecting angle (in the sub-scan sectional plane) of the liquidcrystal element is θ [rad]; and the beam position varying (adjusting)amount on the scanned face is z [mm], the following equation issatisfied:

z=m*Fcol*θ.

[0250] Accordingly, in the case of a light scanning apparatus of whichFcol=15 [mm], m=10 [times], if one desires to adjust the light scanningapparatus with a resolution of Δz=0.001 [mm]=1 [μm], the light pathdeflection by the liquid crystal element 40 is:Δ  θ = z/Fcol * m   = 0.001/15 * 10     = 6.7 × 10⁻⁶[rad]   = 6.7  [μ  rad].

[0251] The liquid crystal element can be driven by a rectangularalternating voltage up to a frequency of several kHz. The light path canbe deflected, that is the outgoing direction of the incident light beamcan be deflected by changing effective voltage of the input signalpulse. FIG. 12A is a schematic diagram for explaining the deflection ofthe light beam by the modulation of the input signal pulse, and FIG. 12Bis a schematic diagram for explaining an example of the reference inputsignal pulse.

[0252] The effective voltage can be generally changed by changing pulsewidth (duty) as shown in FIG. 12C (W->W′), and changing pulse height(A->A′) as shown in FIG. 8D. The refractive index gradient of the liquidcrystal layer, and consequently, the deflecting angle of the light pathcan be controlled by changing the effective voltage as described above.

[0253] For example, in the case that the effective value is changed bychanging the pulse height of the applied voltage, a liquid crystal thatexhibits a light path deflecting angle of 3.0 [mrad] at an effectivevoltage of 2.0 volts, if the pulse height is modulated with a 10 bitcomparator (1024 steps), can deflect the light path at an angle of 3.0[rad]/1024 steps=2.93 [μrad] at an effective voltage of 2.0 [V]/1024steps=1.95 [mV/step] (0.44 [μm] if converted to the beam position on thescanned face).

[0254] Accordingly, one can achieve a precise adjustment in a short timeperiod by coarsely adjusting the light beam position (beam spot pitch)by the mechanical “γ” rotation of the light source unit 18 andsubsequently fine adjusting the light beam position with the liquidcrystal element. Accordingly, a memory unit is provided in the lightscanning apparatus storing the electric signal that drives the liquidcrystal element, and the driving voltage (effective value) that is usedfor fine adjusting is stored in this memory unit. When the lightscanning apparatus is initialized on user's site, for example, the lightscanning apparatus can reproduce the desired light beam position byretrieving the stored effective value from the memory unit. An operatordoes not need to repeat the beam position adjustment on a trial anderror basis at the user's site.

[0255] In the case of the above light source unit 18, two pairs ofsemiconductor lasers 211 a, 211 b and coupling lenses 212 a, 212 b arefixed to the common base member 243. Because the coupling lenses 212 a,212 b are attached to the base member 243 with adhesive material of fromtens to hundreds of μm in thickness, the relative positionalrelationship between the semiconductor lasers and the coupling lensesmay be affected by disturbances after the shipment from the factory. Thedisturbances include any cause that affects the light beam position onthe scanned face including changes over time of material and assembly,vibrations during transportation and installation, and change intemperature and humidity, for example.

[0256] In the case that the relative positional relationship between thesemiconductor lasers and the coupling lenses, especially in the sub scansectional plane, changes, the light beam position on the scanned facechanges. This change causes a change in pitch between two light beams(scan line pitch), which results in the degrading of output images.

[0257] To solve this problem, the change in the light beam position dueto the disturbance is compensated using the above liquid crystal elementto position the light beam with high accuracy.

[0258] In the embodiment described above, a liquid crystal element isprovided to the light path of each light beam. If there are “N” lightbeams, “N” liquid crystal elements are required. Instead, one may regardone light beam as the reference, and provide a liquid crystal element tothe light path of each light beam other than the reference light beam.Only “N−1” liquid crystal elements are required for “N” light beams inthis case. The light beam position of the “N−1” light beams are adjustedto the reference light beam. The number of liquid crystal elementsdecreases.

[0259] A sensor for detecting the light beam position may be provided tothe light scanning apparatus and the image forming apparatus so as todetermine the compensation of the light beam position. If data to berequired for the compensation are available theoretically orexperimentally, a compensation table storing such data may be providedso as to determine the compensation based on the data.

[0260]FIG. 13 is an exploded perspective view showing a light sourceapparatus that emits four light beams. The light source apparatusincludes the following: a first light source unit 241 and a second lightsource unit 242 constructed in the same manner as the light source unit18 shown in FIG. 10; a beam mixing prism 217 that mixes two pairs oflight beams emitted by the light source unit 241 and 242; and a holdingmember (flange) 244 that holds the light source units 241, 242, and thebeam mixing prism 217 as one body. Since the light source apparatus isso constructed, the relative attitude of the first light source unit 241and the second light source unit 242 changes in the sub scan sectionalplane. As a result of the change in attitude, there is a risk that therelative positional relationship between the pair of light beams emittedby the light source unit 241 and the pair of light beams emitted by thelight source unit 242 changes extremely over time or due toenvironmental change such as temperature.

[0261] To avoid the above problem, as shown in FIG. 14A, a liquidcrystal element 240 may be added to the light source apparatus shown inFIG. 13. Only major optical elements of the light source apparatus ofFIG. 14A are shown in FIG. 14B.

[0262] FIGS. 15A-15F are schematic diagrams for explaining six examplesof liquid crystal element 240 shown in FIG. 14A. FIGS. 15A and 15C showsexamples in which a liquid crystal element, or an effective area, isprovided to each of four light beams. The remaining examples showexamples in which a liquid crystal element, or an effective area, isprovided to each of at least three light beams. All examples are thecase where the number of light beams is four (N=4). The effective areais a divisional fraction of the light transmitting unit of a liquidcrystal element. Each effective area is independently driven.

[0263] The examples shown in FIGS. 15A-15F are constructed as follows.

[0264]FIG. 15A shows the case that one liquid crystal element 240 havingfour effective areas a1-a4 corresponding to four light beams isprovided.

[0265]FIG. 15B shows the case that one liquid crystal element 240 havingthree effective areas b2-b4 corresponding to three light beams isprovided. The remaining light beam is the reference beam.

[0266]FIG. 15C shows the case that four liquid crystal elements 240a-240 d corresponding to four light beams, fixed to a common holdingmember 223 are provided.

[0267]FIG. 15D shows the case that thee liquid crystal elements 240b-240 d corresponding to three light beams, fixed to a common holdingmember 223 are provided. The remaining light beam is the reference beam.

[0268]FIG. 15E shows the case that two liquid crystal elements e1 ande2, or a liquid crystal element having two effective areas e1 and e2corresponding to two beams emitted by the first light source unit 241are provided: No liquid crystal element is provided for the two beamsemitted from the second light source unit 242.

[0269]FIG. 15F shows the case that one liquid crystal element isprovided so that the two beams emitted by the first light source unit241 are deflected at the same deflecting angle. No liquid crystalelement is provided for the two beams emitted by the second light sourceunit 242.

[0270] The positional change of the light beams described above can becompensated using the liquid crystal element(s) as shown in FIGS.15A-15F.

[0271] The maximum value of a deflecting angle of a liquid crystalelement that is required by the multi-beam scanning apparatus using afour beam light source apparatus is shown in FIG. 14A.

[0272] In the case of the four beam light source apparatus shown in FIG.14A, for example, the relative deviation in the optical axes of the twobeams emitted from the first light source unit 241 and the two beamsemitted from the second light source unit 242 changes over time or dueto environmental change such as temperature. The relative deviation ofoptical axes between the two pairs of beams is referred to as “relativeoptical axis deviation”. The base members 243 a and 243 b, and theflange 244 are made of metallic material such as iron, aluminum, andzinc, or resin material that is easy to form. According to empiricalknowledge, in the case that the above components are made of suchmaterials, the maximum value of the relative optical axis deviationbecomes up to +/−2.0 [minutes].

[0273] As described above with reference to FIG. 10, the coupling lenses212 a and 212 b are fixed to the holding member via the adhesive layerof tens-hundreds of μm thick. The thickness of the adhesive layer maychange due to thermal expansion, for example, and the change in thethickness may cause the relative deviation of the optical axes of thelight beams.

[0274] Accordingly, the liquid crystal element is desired to have amaximum deviating angle large enough to compensate for this relativedeviation in optical axes.

[0275] On the other hand, as described in the related art, the thicknessof the liquid crystal layer needs to be increased so as to increase themaximum deflecting angle of the liquid crystal element. If the liquidcrystal layer of tens or more μm thick is desired, special spacermembers of high grade and high cost with small variance of diameterswould be required. Increasing the thickness of the liquid crystal layermay cause the following side effects due to the deviation in thicknessof the liquid crystal layer, the transparent electrode (ITO) film, andthe orientation film:

[0276] (a) linear degrading of refractive index distribution;

[0277] (b) change in transmissivity due to multiple interference; and

[0278] (c) degrading of wave-front aberration.

[0279] Accordingly, taking the cost, performance, and yields intoconsideration, the inventor considers it practical to set the maximumdeflecting angle at about +/−4.0 [minutes].

[0280] In the cases shown in FIGS. 15B, 15D, and 15F in which a liquidcrystal element is provided in the light paths of at least “N−1” lightbeams, the relative deviation in the optical axes of +/−2.0 [minute]needs to be compensated. Additionally, taking the adjustment in anassembly process in the factory, the inventor considers that the maximumdeflecting angle of the liquid crystal element need to be+/−2.5[minutes], or preferably, +/−4.0 [minutes].

[0281] On the other hand, in the case that the liquid crystal element isprovided in the light paths of the “N” light beams as shown in FIG. 15E,the relative deviation of +/−2.0 [minutes] in the optical axes betweenthe light beams emitted from the first light source unit and the secondlight source unit is compensable with the two effective areas e1 and e2.Accordingly, the maximum deflecting angle may be about a half themaximum deflecting angle in the cases of FIGS. 15B, 15D, and 15F. If theroom for adjustment in the assembly process is added, the maximumdeflecting angle of the liquid crystal element needs to be+/−1.5[minutes], preferably +/−2.0 [minutes]. Needless to say, in the cases ofFIGS. 15A and 15C, the maximum deflecting angle may be lowered as in thecase of FIG. 15E.

[0282] As described above, since the liquid crystal elements areprovided in the light paths of the “N” light beams, the number of liquidcrystal elements required for the light scanning apparatus may increase.However, because the requisite maximum deflecting angle can be reduced,the requisite optical performance may be achieved at an even lower cost.

[0283] The liquid crystal element according to the above embodiment canadjust the beam position on the scanned face in a desired range withoutdegrading the optical performance even if the maximum deflecting angleis limited to +/−4.0 [minutes].

[0284] An image forming apparatus according to the present invention isdescribed below.

[0285] The image forming apparatus includes a light scanning apparatus,a charging unit, a developing unit, a transferring unit, a fixing unit,a photosensitive body, and a cleaning unit. The light scanning apparatuslight scans the photosensitive body, and forms an electrostatic latentimage on the photosensitive body by electrophotography. The imageforming apparatus and its principle are well known. The photosensitivebody is uniformly charged by the charging unit. As the light scanningapparatus forms an exposure distribution on the scanned face of thephotosensitive body, potential is reduced depending on exposuredistribution. The electrostatic latent image is formed on the scannedface. The developing unit attaches toner on the electrostatic latentimage. The transferring unit transfers the toner attached to thephotosensitive body to a sheet of paper, for example, and the fixingunit fixes the toner on the sheet of paper by fusion adherence. Thecleaning unit removes the toner remaining on the photosensitive body.

[0286] The light scanning apparatus according to an above embodiment ofthe present invention realizes an effect of the present invention, thatis, images are improved by compensating the light beam (beam spot)position on the photosensitive body, depending on necessity. In the caseof the multi-beam scanning apparatus that uses a plurality of beams forscanning, printing speed and printing density can be improved.

[0287] The beam spot pitch mainly in the sub scan directions, that is,the scan line pitch, is given a fine adjustment in the assembly processin the factory, but the scan line pitch may change due to vibration andshock during transportation and installation of the image formingapparatus. The scan line pitch may also change due to change over timeand/or environmental change such as temperature.

[0288] To solve this problem, a detecting system that detects the scanline pitch may be provided to the image forming apparatus. The scan linepitch can be compensated by controlling the liquid crystal element basedon the result of detection.

[0289] The liquid crystal element can change pixel density in the subscan directions. In the case that the image forming apparatus accordingto the present invention is embodied in a multifunctional apparatus thatfunctions as a printer and a copier, for example, the multifunctionalapparatus can switch pixel density in the printer mode in which themultifunctional apparatus functions as a printer and pixel density inthe copier mode in which the multifunctional apparatus functions as acopier. For example, the multifunctional apparatus can switch the pixeldensity between 600 dpi in the printer mode and 400 dpi in the copiermode. The image forming apparatus can realize the pixel density fittingeach mode.

[0290] An operator can switch the pixel density of the image formingapparatus by operating an operational panel, for example, providedthereto. For example, the operator may desire to switch between a highimage quality mode (1200 dpi) and a high speed printing mode (600 dpi).The image forming apparatus according to the present invention caneasily change the pixel density by driving and controlling the lightpath deflecting element provided therein.

[0291] According to the present invention, the light scanning apparatuscan adjust the beam position on the scanned face without degradingoptical performance thereof.

[0292]FIG. 16 is a schematic diagram showing a light scanning apparatusaccording to an embodiment of the present invention, spread in a planeparallel to the deflecting rotative plane. A reference numeral 341refers to a first light source unit; a reference numeral 342 refers to asecond light source unit; a reference numeral 317 refers to a beammixing prism; reference numerals 340 a and 340 b refer to liquid crystalelements that control the positions of light beams on the scanned faceby deflecting the light paths of the light beams from the light sources341 and 342, respectively; a reference numeral 313 refers to acylindrical lens; a reference numeral 314 refers to a polygon mirrorthat is a deflector; a reference numeral 315 refers to a scanningoptical system; a reference numeral 316 refers to a photosensitive drum;a reference numeral 319 refers to a light beam intensity detecting unit;a reference numeral 353 refers to an optical housing; and a referencenumeral 354 refers to the side wall of the housing. The polygon mirror314 is rotated at a constant rotative speed by a driving mechanism (notshown) including a motor.

[0293] A two-beam scanning apparatus that scans two light beamssimultaneously is illustrated as an example of a light scanningapparatus used for a color image forming apparatus. The light scanningapparatus according to the present invention is also applicable to amulti-beam scanning apparatus that scans more than two light beams.

[0294] Two light beams 321 a and 321 b emitted by the first light sourceunit 341 and the second light source unit 342, respectively, are mixedby the beam mixing prism 317, then converge and form a long linear imagein the main-scan directions on the deflecting reflective face of thepolygon mirror 314 through the cylindrical lens 313. The light beamsforms beam spots on the surface of the photosensitive body drum 316 bythe scanning optical system (scanning lens) 315 and scan the surface.The main-scan directions are directions in which the beam spots scan thesurface of the photosensitive body drum 316. The sub-scan directions aredirections perpendicular to the main-scan directions. In the followingdescription, the directions corresponding to the main-scan directionsand the sub-scan directions at a position in the light paths are calledin a broad sense, “the main-scan directions” and “the sub-scandirections”, respectively.

[0295] A light beam position compensating unit is often provided to theabove light scanning apparatus (multi-beam scanning apparatus) so as toinitially adjust the light beam position (light beam pitch) and tocompensate environmental and change over time.

[0296]FIG. 17 is an optical layout showing an image forming apparatusaccording to an embodiment. The image forming apparatus 3200 is a colorimage forming apparatus that outputs color images. The image formingapparatus 3200 is a so-called tandem type image forming apparatus inwhich four light scanning apparatus 320K, 320C, 320M, and 320Y identicalto the light scanning apparatus 320 shown in FIG. 16 are used as lightwriting apparatuses. The light beam position compensating unit is oftenprovided to the tandem type image forming apparatus so as to adjust thelight beam positions between the light writing apparatuses (stations).The image forming apparatus 3200 is described in detail below.

[0297] Light beam position compensating unit are conventionallyconstructed in the following manner, in which the light beam isdeflectable:

[0298] (1) rotating the reflecting mirror,

[0299] (2) shifting or rotating the cylindrical lens,

[0300] (3) shifting or rotating the prism,

[0301] (4) providing an electro-optical element or an AOM, and

[0302] (5) rotating a parallel plate provided between the semiconductorlaser and the coupling lens.

[0303] The above conventional methods cause problems such that the size,power consumption, heat production, and noise of the system become toolarge. To solve these problems, a liquid crystal element of which size,weight, and power consumption are small and that produces no noise isemployed as the light beam position compensating unit.

[0304] The transmissivity of the liquid crystal element depends on thecompensation amount of the light beam position, that is, beam deflectingangle. The change in the transmissivity of the liquid crystal elementmay cause a change in light intensity at the surface of thephotosensitive drum 316, and the change in the light intensity maydegrade images formed by the image forming apparatus. Accordingly,according to the present invention, the change in transmissivity of theliquid crystal due to the change in the beam deflecting angle isspecified to be 4% or less, more preferably 2% or less so as to make thechange in the light beam intensity at the scanned face small andconsequently, to prevent the quality of images from degrading.

[0305] The change in the light beam intensity at the scanned face meansthe change in the light beam intensity at the same image height. Thechange does not include “shading properties” depending on the imageheight, that is, the change in the reflective index of the polygonmirror due to the rotation thereof, the change in transmissivity andreflective index of the scanning lens and the reflecting mirror, forexample, depending on the image height.

[0306] Where the focal distance of the coupling lens fcol=15 (mm), andthe sub-scan convergence magnification of the entire multi-beam scanningapparatus mZ=9.5 (times), and the light path deflecting angle of theliquid crystal element θ=2.0 (minutes)=0.66 (mrad), the change in thelight beam position at the scanned face is:Z = m  Z * fcol * tan   θ   = 9.5 * 15 * tan   (0.66 × 10⁻³)   = 0.095  (mm) = 95(μm).

[0307] In the case of an optical system having the above opticalproperties, for example, the change in the light beam position at thescanned face due to change over time, environmental change, andinstallation is empirically known to be about 10 μm. The light pathdeflecting angle required for compensating the change in the light beamposition is at most θ=2.0 min.

[0308] The change in transmissivity of the liquid crystal element withinthe above light path deflecting angle is an extremely importantparameter to maintain both the light beam position and the light beamintensity at high accuracy. The change in light beam intensity caused bythe light beam position adjustment within the required range iscontrollable provided that T/θ is equal to or less than 4 (%)/2.0(min)=2.0 (%/min), where the change in transmissivity of the liquidcrystal is T (%), and the light path deflecting angle is θ (minute).

[0309] The reasons why the transmissivity of the liquid crystal elementchanges, and consequently the light beam intensity on the scanned facechanges are as follows:

[0310] (1) The nematic type liquid crystal element forms the gradient ofrefractive index in the liquid crystal layer and consequently deflectsthe light beam by changing the anisotropy in refractive indexes ofregular ray and irregular ray Δn (ne−no) with driving voltage applied tothe liquid crystal. As shown in FIG. 18B, the transmissivity cyclicallychanges as a function of the beam deflecting angle, that is, theanisotropy in refractive index. The anisotropy Δn can be changedgenerally by applying a voltage to the liquid crystal layer sandwichedby a pair of glass substrates, transparent electrodes, and orientationfilms, for example, through the transparent electrodes. The gradient ofrefractive index proportional to the applied voltage is formed in theliquid crystal layer.

[0311] (2) A liquid crystal element can deflect the light beam bydiffracting the light beam with a diffractive lattice based on thepattern of electrodes. The transmissivity changes since the diffractiveefficiency of the diffractive lattice depends on the pitch of thediffractive lattice.

[0312]FIGS. 3A and 3B are graph of the light path deflecting angle andthe transmissivity, respectively, as functions of a parameteru=2*Δn*d*X, where Δn (=ne−no) is the anisotropy in refractive index ofthe liquid crystal, “d” is the thickness of the liquid crystal layer,and “λ” is the wavelength of the light beam. The parameter “u”corresponds to the deflecting angle φ of the light beam by the liquidcrystal element.

[0313] As described above, in the case of the optical system satisfyingthe previous conditions, that is, fcol=15 (mm), and mZ=9.5, the range inwhich the transmissivity η changes is desired to be 4%, more preferably2%. Accordingly, the liquid crystal needs to be used in the region “A”shown in FIG. 18B in which the transmissivity is between 98% and 100%(width being 2%). However, as shown in FIG. 18A, the wavefrontaberration (spherical aberration) becomes too large and the opticalproperties are degraded in a region “B”. The liquid crystal deflectingelement is not usable in the region “B”.

[0314] The transmissivity T of the liquid crystal is obtainable usingthe following equation:

η=1−[sin² {(π/2)×(1+u ²)^(1/2)}]/(1+u ²).

[0315] If the light beam position is desired to be moved by Δz in thesub-scan directions on the scanned face, the light beam needs to bedeflected by φZ (the sub-scan directions component of the beamdeflecting angle) so that φZ satisfies

Δz=fcol×mZ×tan φZ, or

φZ=tan⁻¹(Δz/fcol×mZ),

[0316] where fcol is the focal distance of the coupling lens, mZ is thesub-scan magnification of the entire system, and φZ is the sub-scandirections component of the beam deflecting angle.

[0317] If the light beam position is desired to be moved by Δy in themain-scan directions on the scanned face, the light beam needs to bedeflected by φY so that φY satisfies

φY=tan⁻¹(Δy/fcol×mY)=tan⁻¹(Δy/FY),

[0318] where mY is the main-scan magnification of the entire system, andφY is the main-scan directions component of the beam deflecting angle.

[0319]FIG. 19 is another graph of the transmissivity of the liquidcrystal element as a function of the parameter u=2*Δn*d*λ. If thetransmissivity of the liquid crystal element changes as shown in FIG.19, the liquid crystal element needs to be used in regions A1-An inwhich the transmissivity changes in a range between 91% and 93% (widthbeing 2%). Since the regions A1-An are separate from each other, theusable light path deflecting angles are discrete. In this case, one mayshorten the cycle by setting design parameters appropriately so as toobtain the desired deflecting angle at a high resolution. However,because the wavefront aberration of the liquid crystal becomes large asthe parameter “u” increases, the liquid crystal element cannot be usedin the regions on the right side of the region A_(n+1) for example.

[0320] In the case of the light scanning apparatus according to thepresent invention, 10 or more cycles of the cyclic change intransmissivity may exist in the range of the light path deflection, thatis, the range of the light beam position adjustment. For example, in thecase of the above optical system where fcol=15 (mm) and mZ=9.5, if thelight beam position on the scanned face is desired to be moved by 95(μm), and the cycle of the cyclic change in transmissivity is set at 10or more, the light beam position becomes adjustable at a resolution ofat least 9.5 (μm).

[0321] If the maximum value of the transmissivity in each cycle issubstantially equal, the transmissivity can be maintained at high side,and energy loss can be reduced.

[0322] Additionally, in the case that the transmissivity decreases whileoscillating as the anisotropy Δn increases as shown in FIG. 20, theliquid crystal is usable in a region in which the beam deflecting anglefalls into the desired range and the transmissivity change is 4% (morepreferably 2%). In the case of FIG. 20, the liquid crystal element isused in the region “A” where the transmissivity is between 94% and 96%(width being 2%).

[0323] As shown in FIG. 18A, in general, the light path deflecting anglelinearly increases as the voltage applied to the liquid crystal layerincreases, that is, Δn increases. The linearity, however, is lost as theapplied voltage further increases and exceeds a certain value. Thewavefront aberration becomes too large too. Such a region of the appliedvoltage, that is, the parameter “u”, is shown in FIG. 18A as the region“B”.

[0324] The liquid crystal element is difficult to use in a nonlinearregion on the left side of the linear region shown in FIG. 18A. Thepreferably designed liquid crystal elements exhibit a wide region inwhich their transmissivity is 2% or less and, at the same time, thelight path deflecting angle changes linearly.

[0325] As described above, the deviation in the light beam at thescanned face, that is, at the same image height and/or the deviationbetween a plurality of beams is reduced by the reduction of thetransmissivity change in the desired range (light beam positionadjusting range) of the light path deflection angle of the liquidcrystal element. Accordingly, even when switching the light beam fromone color to another, the light scanning apparatus according to thepresent invention can avoid imbalance among the colors and consequentlyavoid the degrading of color image quality.

[0326] In addition, a light beam intensity detecting unit may beprovided to the light scanning apparatus according to the presentinvention. The light beam intensity detecting unit detects the lightbeam intensity at the scanned face or a plane equivalent thereto. Whenthe light beam intensity exceeds a predetermined value, the liquidcrystal element may be driven to deflect the light path so that thelight beam intensity falls in a predetermined range.

[0327] In the case that the liquid crystal element exhibits thetransmissivity as shown in FIG. 19, the adjustment of the light beamposition may be discrete. This problem can be solved by appropriatelydesigning the cycle of the transmissivity oscillation based on thedesired resolution of the light beam position adjustment.

[0328] The light beam intensity or the deviation in intensity between aplurality of light beams may be further improved (1% or less, forexample) by the light beam intensity compensating unit that compensatesthe light beam intensity on the scanned face.

[0329] As shown in FIG. 18A, the relationship between the light pathdeflecting angle of the liquid crystal element and the change in thelight beam intensity at the scanned face is theoretically determinable(by design). If a table with which the light intensity compensatingamount can be obtained based on the beam deflecting amount is prepared,and the light beam intensity compensating unit is driven/controlledbased on such a table, the light scanning apparatus can compensate thelight beam intensity without the light beam intensity detecting unit.

[0330] The emission intensity of the light source (semiconductor laser)may be controlled in order to compensate the light beam intensity.Alternatively, a filter such that controls the light beam intensityusing the polarization characteristics of the light beam, absorbs lightenergy inside, or reflects the light energy at the surface thereof maybe provided in the light path of the light beam.

[0331] The actual intensity of the light beam may vary due to dispersionof parts and assembly, change over time, and environmental change suchas temperature and humidity. Accordingly, it is recommended that, asshown in FIG. 16, a light beam intensity detecting unit 319 be providedon the scanned face (or at an optically equivalent position) so that thelight beam intensity compensating unit is controlled based on the lightbeam intensity detected by the light beam intensity detecting unit 19.

[0332] An image forming apparatus according to the present invention isdescribed below.

[0333]FIG. 17 is a optical layout showing an image forming apparatusaccording to the embodiment. The image forming apparatus 3200 includesthe following: the light scanning apparatuses 320K, 320C, 320M, and 320Ycorresponding to black (K), cyan (C), magenta (M), and yellow (Y),respectively; the photosensitive drums 316K, 316C, 316M, and 316Y; thedeveloping unit that manifests electrostatic latent images with tonerformed on the surface of the photosensitive drums; the transferring unitthat transfers the manifest toner images; and other units for processingthe electrophotography process. The image forming apparatus 3200 is atandem type image forming apparatus that advantageously forms highdensity color images at a high speed by scanning a plurality of lightbeams simultaneously. The surface of each photosensitive body drumcorresponds to the scanned face. Compared with a color image formingapparatus having one exposing unit that needs to form an imagecorresponding to a different color four times, the color image formingapparatus having four exposing units can output images four times involume. The developing unit and the transferring unit, for example, arenot shown in the drawing.

[0334] Although not shown in FIG. 17, a processing unit forelectrophotography such as a charging unit, a developing unit, and atransferring unit, are provided around each photosensitive drum. Thelight scanning apparatus performs the exposure process ofelectrophotography, and specifically, forms electrostatic latent imagesby scanning the surface of the photosensitive drum uniformly charged bythe charging unit.

[0335] In the multi-color mode of the tandem type image formingapparatus having a plurality of photosensitive bodies, electrostaticlatent images are formed on the photosensitive body drums 316K, 316C,316M, and 316Y by exposing units in response to an image signal ofcorresponding color.

[0336] The electrostatic latent images are developed with a toner ofcorresponding color into toner images, and sequentially transferred toand superposed on a sheet of transfer paper electrostatically absorbedand carried by a transfer belt. The superposed images are fixed anddischarged by the fixing unit as a full color image. The transferringbelt, the transfer paper, and the fixing unit are not shown in thedrawing.

[0337] In a single color mode, however, only the photosensitive bodydrum and the processing units corresponding to a color (either K, C, M,or Y) operate, and the photosensitive body drums and the processingunits corresponding to the other colors do not operate.

[0338] In the case that a light beam is emitted from each light scanningapparatus 320K, 320C, 320M, and 320Y, the image forming apparatus canform full-color (four color) images. In the case that at least one ofthe four light scanning apparatuses is the light scanning apparatus witha plurality of light beams (N), and the light scanning is performed byonly this light scanning apparatus, an image “N” times as high indensity as the full color image is available.

[0339] If the carrying speed of recording medium and processing speed isset to four times, the number of output pages becomes four times.Additionally, even in the full color image mode, characters are oftenwritten in black at a high density. While the above multi-beam (N) lightscanning apparatus 320K (black) operates, the other (single beam) lightscanning apparatuses 320C, 320M, and 320Y are operated. Accordingly, theimage forming apparatus can output high-quality image with characters,photographs, and drawings mixed.

[0340] The light scanning apparatus according to an embodiment of thepresent invention provided in the image forming apparatus 3200 canadjust the relative position (beam pitch) of the plurality of beamsscanning the surface of the photosensitive body drum (scanned face) to adesired value. Accordingly, the image forming apparatus 3200 can formhigh quality images.

[0341]FIG. 21 is a schematic diagram showing an image forming apparatusaccording to another embodiment of the present invention. The imageforming apparatus shown in FIG. 21 is a tandem type image formingapparatus in which a plurality of light beams emitted by a plurality oflight sources scan a plurality of scanned faces simultaneously. Apolygon mirror and a scanning optical system as a deflecting unit arenot described since they are identical to those known in public. Aliquid crystal element 340 is provided in each light path to thephotosensitive body drum 316K, 316C, 316M, and 316Y, the liquid crystalelement 340 adjusting the light beam position on each photosensitivebody drum. The liquid crystal element 340 may be a single elementincluding a plurality of effective areas therein, or may be made of aplurality of independent elements corresponding to the light beams eachhaving a single effective area.

[0342] The light scanning apparatus according to an embodiment of thepresent invention provided in the image forming apparatus can compensatethe relative position of the light beams scanning differentphotosensitive body drums with the liquid crystal element 340. Forexample, a toner image 332 formed on a transfer belt 331 is detected bya color mismatch detecting sensor 333, and the liquid crystal element340 is driven based on the result of the detection, that is, the colormismatch between stations; the image forming apparatus can compensatethe difference in timing at which each station starts writing.Accordingly, the color mismatch of the toner image 332 on thetransferring belt 331 is reduced, which results in high quality images.

[0343] If an image forming apparatus such as a copier or a printercontinuously outputs tens or more of sheets of transfer paper, a colormismatch between stations may occur due to the increase in temperaturein the image forming apparatus. When a liquid crystal element is used tocompensate the color mismatch, the transmissivity may change dependingon the compensation amount, and as a result, the light beam intensity onthe photosensitivity body drum may change. The light scanning apparatusaccording to the present invention, however, can reduce the change inthe light beam intensity. Accordingly, the light scanning apparatus canprevent the quality of output images from degrading even when continuousimage forming is required.

[0344] In addition, the light scanning apparatus can prevent the qualityof output images from degrading due to shock and vibration duringtransportation and installation of the image forming apparatus and/orchange over time.

[0345] It is not necessary to provide a liquid crystal element in alllight paths of the light beams. If a color (black, for example) is usedas a reference, the liquid crystal element may be provided in the lightpaths of the other colors (cyan, magenta, and yellow, for example) so asto adjust the light beam positions to the reference color.

[0346]FIG. 22 is a schematic diagram showing an image forming apparatusaccording to yet another embodiment of the present invention. FIG. 22shows the layout of the optical components in a plane parallel to thedeflecting rotation plane. This image forming apparatus includes aplurality of light scanning apparatuses 320 arrayed in the main scandirections. A scanned face 316 is divided into a plurality of regions bydividing the effective writing width thereof, and each light scanningapparatus 320 scans a corresponding region.

[0347] The light scanning apparatus according to the present invention,if applied to the image forming apparatus, can compensate the light beamposition at the joint of the divided scan on the scanned face 316 atinstallation, for example. If the effective writing width can beincreased, optical components and deflecting units, for example,constructing the light scanning apparatus can be downsized; the changein beam waist position due to mechanical tolerance and temperaturechange becomes small, and the wavefront aberration is reduced.Accordingly, the image forming apparatus can output high quality images.

[0348] According to the present invention, the transmissivity of theliquid crystal element that controls the light beam position on thescanned face within the deflecting range of the light path issuppressed, the deviation between a plurality of light beams on thescanned face, and the degrading in quality of output image can besustained.

[0349] The preferred embodiments of the present invention are describedabove. The present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

[0350] This patent application is based on Japanese priority patentapplications No. 2002-204164 filed on Jul. 12, 2002, No. 2002-379681filed on Dec. 27, 2002, and No. 2002-379958 file on Dec. 27, 2002, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A light scanning apparatus that scans a scannedface with a light beam, comprising: an adjusting unit that adjusts theposition of a light spot of said light beam formed on the scanned face;and a compensating unit that compensates the light intensity of saidlight beam at said scanned face due to change caused by the adjustmentof the position of said light spot.
 2. The light scanning apparatus asclaimed in claim 1, wherein said light scanning apparatus scans saidscanned face with a plurality of (N) light beams emitted by “N”, lightsources; said adjusting unit further comprises at least “N−1” deflectingunits located between said light source and a scanning unit, whereineach of the deflecting units deflects a corresponding one of theplurality of light beams in sub-scan directions and adjusts scan linepitch.
 3. The light scanning apparatus as claimed in claim 2, whereinthe deflecting units are liquid crystal deflecting elements.
 4. Thelight scanning apparatus as claimed in claim 2, wherein said deflectingunit further comprises a semiconductor laser and a coupling lenscombined with a holder rotatable around an axis parallel to the opticalaxis of said coupling lens, the emission source of said semiconductorlaser being eccentric to said optical axis.
 5. The light scanningapparatus as claimed in claim 4, wherein said deflecting unit furthercomprises an aperture combined with said holder that shapes said lightbeam, said aperture being eccentric to the light path of said light beamemitted by said semiconductor laser and passing through the center ofsaid coupling lens.
 6. The light scanning apparatus as claimed in claim1, wherein said adjusting unit further comprises a liquid crystaldeflecting element array having a plurality of liquid crystal deflectingelements arrayed in main-scan directions, each of which deflects saidlight beam in sub-scan directions, said liquid crystal deflectingelement array being provided between said scanning unit and said scannedface.
 7. The light scanning apparatus as claimed in claim 1, furthercomprising a detecting unit that detects the intensity of said lightbeam.
 8. The light scanning apparatus as claimed in claim 7, whereinsaid detecting unit further detects said light beam for synchronizationof light scanning.
 9. The light scanning apparatus as claimed in claim1, wherein said compensating unit controls the radiation intensity ofsaid light source.
 10. The light scanning apparatus as claimed in claim1, further comprising an aperture, provided between said light sourceand said scanning unit, that shapes said light beam; wherein saidcompensating unit displaces said aperture.
 11. The light scanningapparatus as claimed in claim 1, wherein said compensating unit controlsa transmissivity adjusting unit provided between said light source andsaid scanning unit.
 12. The light scanning apparatus as claimed in claim1, further comprising a resin lens provided in the optical path fromsaid light source to said scanned face.
 13. An image forming apparatus,comprising: a photosensitive medium; and a light scanning apparatus thatscans said photosensitive medium with a light beam; wherein said lightscanning apparatus further comprises: an adjusting unit that adjusts theposition of a light spot of said light beam formed on saidphotosensitive medium; and a compensating unit that compensates thelight intensity of said light beam at said photosensitive medium due tochange caused by the adjustment of said position of said light spot. 14.The image forming apparatus as claimed in claim 13, wherein saidphotosensitive medium is a photoconductive photosensitive body; and anelectrostatic latent image formed by the light scanning is made visibleby being converted into a toner image.
 15. The image forming apparatusas claimed in claim 14, wherein said light scanning apparatus scans saidphotoconductive photosensitive body with a plurality of (N) light beamsemitted by “N” light sources; said adjusting unit further comprises atleast “N−1” deflecting units located between said light source and ascanning unit, wherein each of the deflecting units deflects acorresponding one of the plurality of light beams in sub-scan directionsand adjusts scan line pitch.
 16. The image forming apparatus as claimedin claim 13, wherein said image forming apparatus is a tandem type inwhich one or more photosensitive bodies that are drum-shaped orbelt-shaped are provided along the path of a toner image medium, and atoner image formed on each photosensitive body is transferred to saidtoner image medium generating a composite color image.
 17. The imageforming apparatus as claimed in claim 16, wherein four photosensitivebodies are provided corresponding to magenta, cyan, yellow, and black;or three photosensitive bodies are provided corresponding to red, green,and blue.
 18. A method of scanning a scanned face with a light beam,comprising the steps of: emitting, by a light source, said light beam;deflecting, by a scanning unit, the emitted light beam; and converging,by a converging unit, the deflected light beam forming a light spot;wherein the position of said light spot formed by the converged lightbeam on said scanned face is adjustable by an adjusting unit; and thelight intensity of said light beam at said scanned face due to changecaused by the adjustment of the position of said light spot iscompensable by a compensating unit.
 19. The method as claimed in claim18, wherein said adjusting unit is provided between said light sourceand said scanning unit, and adjusts scan line pitch of light scanningwith a multi-beam scanning method.
 20. The method as claimed in claim18, wherein said adjusting unit is provided between said scanning unitand said scanned face, and compensates the curvature of a scan line. 21.A light scanning apparatus that scans a scanned face with a plurality of(N) light beams, comprising a plurality of adjusting units, each ofwhich adjusts the position of a scan line formed by a corresponding oneof the plurality of light beams; wherein at least one of the pluralityof adjusting units is a liquid crystal element driven by an electricsignal.
 22. The light scanning apparatus as claimed in claim 21, furthercomprising a memory unit that stores said electric signal driving saidliquid crystal element.
 23. The light scanning apparatus as claimed inclaim 22, wherein said liquid crystal element initially adjusts theposition of said scan line in compliance with said electrical signalstored in said memory unit.
 24. The light scanning apparatus as claimedin claim 21, wherein said liquid crystal element adjusts the position ofthe light beam due to change caused by an external disturbance.
 25. Thelight scanning apparatus as claimed in claim 21, said liquid crystalelement being able to deflect said light beam by a micro angle.
 26. Thelight scanning apparatus as claimed in claim 21, wherein at least “N−1”of the plurality of adjusting units are liquid crystal elements.
 27. Thelight scanning apparatus as claimed in claim 26, wherein a maximumdeflecting angle of each liquid crystal element is +/−4.0 (minute) orless.
 28. The light scanning apparatus as claimed in claim 21, whereinthe plurality of adjusting units are liquid crystal elements of which amaximum deflecting angle is +/−2.0 (minute).
 29. An image formingapparatus, comprising: a plurality of scanned faces; and a lightscanning apparatus that scans the plurality of scanned faces with aplurality of (N) light beams and forms an electrostatic latent image oneach of the plurality of scanned faces; wherein said light scanningapparatus further comprises a plurality of adjusting units, each ofwhich adjusts the position of a scan line formed by a corresponding oneof the plurality of light beams; and at least one of the plurality ofadjusting units is a liquid crystal element driven by an electricsignal.
 30. The image forming apparatus as claimed in claim 29, whereinsaid liquid crystal element can change pixel density in sub-scandirections.
 31. A light scanning apparatus, comprising a liquid crystalelement that deflects a light beam from a light source to adjust theposition of a light spot formed by said light beam on a scanned face;wherein the ratio of a change in transmissivity (%) of said liquidcrystal element caused by the deflection to a deflecting angle (minute)is equal to or smaller than 2.0 (%/minute).
 32. The light scanningapparatus as claimed in claim 31, wherein said ratio is equal to orsmaller than 2.0 (%/minute) in 10 or more ranges of said deflectingangle, said ranges appearing cyclically.
 33. The light scanningapparatus as claimed in claim 31, further comprising a detecting unitthat detects the intensity of said light beam on said scanned face. 34.The light scanning apparatus as claimed in claim 31, further comprisinga compensating unit that compensates the intensity of said light beam onsaid scanned face.
 35. An image forming apparatus, comprising: a scannedface; and a light scanning apparatus that scans said scanned face with alight beam and forms an electrostatic latent image on said scanned face;wherein said light scanning apparatus further comprises a liquid crystalelement that deflects said light beam from a light source to adjust theposition of a light spot formed by said light beam on said scanned face;and the ratio of a change in transmissivity (%) of said liquid crystalelement caused by the deflection to a deflecting angle (minute) is equalto or smaller than 2.0 (%/minute).